require ruby 2.3 as minimum version

[lsylvester]

In https://github.com/cerebris/jsonapi-resources/pull/1188, support for ruby2.1 and 2.2 in the CI was removed.

When trying to use this gem with ruby 2.1, I am getting the following error:

jsonapi-resources-0.9.10/lib/jsonapi/resource_serializer.rb:100: syntax error, unexpected ':', expecting => (SyntaxError)
      rel_hash = { 'data': data }

This PR updates the required ruby version so that jsonapi-resources can't be installed on ruby versions that is does not support.

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[lsylvester]

Bundler 2 was being installed by default for some ruby versions, which caused bundle install to fail as bundler didn't match what was in the gem spec (this looks like it cause failures on other PRs as well).

I have updated the travis.yml to uninstall bundler 2 and install bundler 1.X - as it is required for rails 4.2 support.

[lgebhardt]

@lsylvester Thanks! I ended up fixing the bundler issue in an earlier PR so I just add the ruby 2.3 minimum version commit. The way I handled bundler isn't exactly the same, but it seems to work. If you think we still need to uninstall versions let's make a new PR for that.