Travis CI. Fix specific bundler version installation

[andrykonchin]

Specify activemodel instead of rails in specs.

There is an issue with running specs against Rails 4.2 because this version requires bundler < 2.0.

Travis CI provides Ruby with preinstalled Bundler and makes it default gem (so you cannot uninstall it). After releasing Bundler 2 TravisCI preinstalls it and it breaks our specs.

As far as only Rails limits bundler version but not activemodel and Dynamoid depends on activemodel only lets get rid of Rails in specs

[pboling]

Cool!

[pboling]

don't require the whole rails gem but only activemodel (because only rails gem limits bundler version) in specs

This would be a great step forward, as it would also allow people using activemodel outside Rails, like in a Sinatra app, to use Dynamoid. It may require some refactoring away from some rails-isms though. I think that's generally a positive move.

As a temporary fix I think skipping JRuby in the build is fine.

[andrykonchin]

@pboling Actually Dynamoid already depends on activemodel only (https://github.com/Dynamoid/dynamoid/blob/master/dynamoid.gemspec#L43)

Rails is required only in test environment on CI so we could drop it right now.