Use fork of mongoid_rails_migrations to fix build

[floehopper]

• This is an attempt to fix the problem described in Tests fail with new bundler on CI #816. It uses a forked version of mongoid_rails_migrations. The branch in that fork was branched off the v1.0.0 tag, which is the version of the gem that was specified in our Gemfile. This is so that we keep the changes to our app to a minimum.

• This fix is a bit of a bodge - the proper fix would be along the lines of Don't require/use Bundler at runtime adacosta/mongoid_rails_migrations#3, but given that the failing build is a significant blocker, it feels as if this is worth a try. If it works OK, we can work on getting the proper fix in place.

• I installed v1.14.5 of Bundler which is the version now specified in govuk-puppet for Ruby v2.2.4. Hence the change to BUNDLED WITH in the Gemfile.

• I've run a migration locally to ensure that the gem still works OK even with this patch in place.

[floehopper]

Note that the newer version of Bundler seems to cause a bunch of warnings when installing the bundle from scratch:

Unable to use the platform-specific () version of activesupport (3.2.22.3) because it has different dependencies from the ruby version. To use the platform-specific version of the gem, run `bundle config specific_platform true` and install again.
Unable to use the platform-specific () version of sprockets (2.2.3) because it has different dependencies from the ruby version. To use the platform-specific version of the gem, run `bundle config specific_platform true` and install again.
Unable to use the platform-specific () version of cucumber (2.2.0) because it has different dependencies from the ruby version. To use the platform-specific version of the gem, run `bundle config specific_platform true` and install again.
Unable to use the platform-specific () version of railties (3.2.22.3) because it has different dependencies from the ruby version. To use the platform-specific version of the gem, run `bundle config specific_platform true` and install again.
Unable to use the platform-specific () version of cucumber-rails (1.4.5) because it has different dependencies from the ruby version. To use the platform-specific version of the gem, run `bundle config specific_platform true` and install again.
Unable to use the platform-specific () version of domain_name (0.5.20161129) because it has different dependencies from the ruby version. To use the platform-specific version of the gem, run `bundle config specific_platform true` and install again.
Unable to use the platform-specific () version of faraday (0.9.0) because it has different dependencies from the ruby version. To use the platform-specific version of the gem, run `bundle config specific_platform true` and install again.
Unable to use the platform-specific () version of rest-client (2.0.0) because it has different dependencies from the ruby version. To use the platform-specific version of the gem, run `bundle config specific_platform true` and install again.
Unable to use the platform-specific () version of oauth2 (1.3.0) because it has different dependencies from the ruby version. To use the platform-specific version of the gem, run `bundle config specific_platform true` and install again.
Unable to use the platform-specific () version of omniauth (1.4.2) because it has different dependencies from the ruby version. To use the platform-specific version of the gem, run `bundle config specific_platform true` and install again.
Unable to use the platform-specific () version of rubocop (0.39.0) because it has different dependencies from the ruby version. To use the platform-specific version of the gem, run `bundle config specific_platform true` and install again.
Unable to use the platform-specific () version of jquery-rails (3.1.4) because it has different dependencies from the ruby version. To use the platform-specific version of the gem, run `bundle config specific_platform true` and install again.
Unable to use the platform-specific () version of jasmine-rails (0.14.1) because it has different dependencies from the ruby version. To use the platform-specific version of the gem, run `bundle config specific_platform true` and install again.
Unable to use the platform-specific () version of quiet_assets (1.0.3) because it has different dependencies from the ruby version. To use the platform-specific version of the gem, run `bundle config specific_platform true` and install again.
Unable to use the platform-specific () version of redis-namespace (1.5.0) because it has different dependencies from the ruby version. To use the platform-specific version of the gem, run `bundle config specific_platform true` and install again.
Unable to use the platform-specific () version of rspec-expectations (3.2.1) because it has different dependencies from the ruby version. To use the platform-specific version of the gem, run `bundle config specific_platform true` and install again.
Unable to use the platform-specific () version of rspec-mocks (3.2.1) because it has different dependencies from the ruby version. To use the platform-specific version of the gem, run `bundle config specific_platform true` and install again.
Unable to use the platform-specific () version of rspec-rails (3.2.3) because it has different dependencies from the ruby version. To use the platform-specific version of the gem, run `bundle config specific_platform true` and install again.
Unable to use the platform-specific () version of sinatra (1.4.4) because it has different dependencies from the ruby version. To use the platform-specific version of the gem, run `bundle config specific_platform true` and install again.

However, as far as I can tell those warnings don't seem to be causing any problems at the moment.

[floehopper]

The build is passing. Yay! 🎉

[h-lame]

A passing build unblocks us and that's what's important for now I think. Good sleuthing @floehopper! I reckon I was on a parallel track with what I found in #816, but this has an actual solution which is better than my "well, we'll have to get bundler to release a new version and upgrade all our machines again" option.