Specify supported rails versions

[zubin]

Context

Currently it's unclear which versions of rails are supported.

After installing this repo, the build failed mysteriously with:

NoMethodError:
  undefined method `configure' for #<TestApp::Application:0x00007ff52a0fab78>
  Did you mean?  config
# ./spec/test_app/config/environments/test.rb:1:in `<top (required)>'

This is because I had rails 4.0 installed, which satisfied the gem requirements but wasn't compatible with the test app.

Changes

• Add appraisal gem for testing against multiple rails versions.
• Update CI script to use appraisal.

Considerations

• The gems are vendored into gemfiles. See here for rationale.
• Only rails 4.2 is tested because the test app appears to be generated with that version.

[zubin]

@shevaun Added some commits in preparation for rails 5 support.

[shevaun]

LGTM