Fix the travis CI build

The travisCI build is breaking because the tests are being done against an old version of hiera-eyaml (1.3.4) that is references in the Gemfile.lock.

The root problem is that the Gemfile contains a dependency on hiera-eyaml-plaintext. This is used to test a whole bunch of the plugin features, verifying that the plugin integration is working. However, it depends on hiera-eyaml and by pulling in a version of the gem we end up running our tests against the gem instead of the source code in the project (this is bad).

The solution I've agreed with @gtmtech to remove the dependency for the plaintext gem.