Avoid calling Bundler.require in library code

This was causing us problems [1] in an application which uses Bundler
when we upgraded to Bundler >= v1.13.0.

I think the right solution is something along the lines of adacosta#3, but I'm
trying this as a temporary quick fix.

Note that this branch is based off the v1.0.0 tag which is the version
of the gem that we are currently using in our app.

[1]: alphagov/manuals-publisher#816

lib/mongoid_rails_migrations.rb

@@ -1,7 +1,4 @@
 # encoding: utf-8
-require 'bundler/setup'
-Bundler.require(:mongoid_rails_migrations)
-
 # Add base to path incase not included as a gem
 $:.unshift(File.dirname(__FILE__)) unless
   $:.include?(File.dirname(__FILE__)) || $:.include?(File.expand_path(File.dirname(__FILE__)))
@@ -10,4 +7,4 @@
 require 'mongoid_rails_migrations/models/data_migration'
 require 'mongoid_rails_migrations/mongoid_ext/mongoid'
 require 'mongoid_rails_migrations/mongoid_ext/railtie'
-require 'mongoid_rails_migrations/active_record_ext/migrations'
+require 'mongoid_rails_migrations/active_record_ext/migrations'