Allow "ruby file:" syntax in gemfile

[swelther]

Given in the gemfile the following ruby syntax is used:

ruby file: "../.ruby-version"

…

Appraisal would fail with wrong number of arguments (given 0, expected 1) (ArgumentError)

…/appraisal-2.5.0/lib/appraisal/bundler_dsl.rb:66:in `ruby': wrong number of arguments (given 0, expected 1) (ArgumentError)
	from …/appraisal-2.5.0/lib/appraisal/gemfile.rb:27:in `run'
	from …/appraisal-2.5.0/lib/appraisal/gemfile.rb:19:in `instance_eval'
	from …/appraisal-2.5.0/lib/appraisal/gemfile.rb:19:in `run'
	from …/appraisal-2.5.0/lib/appraisal/gemfile.rb:25:in `block in dup'
…

This PR fixes this issue by allowing keyword arguments.

There is just one "visual" downsize that the syntax in the appraisal gemfiles is converted to Ruby 1.8 syntax:

-ruby file: ".ruby-version"
+ruby {:file=>".ruby-version"}

Not sure if this is a big issue. Could be probably ignored because it still works and is just some minor visual thing.

[swelther]

Hm seems I opened the PR too fast, in a real example the ruby line is omitted. Need to check why or if only my integration is somehow wrong :)

[swelther]

I didn't find the already existing spec for this feature, adapted the code to work now. Just wondering why the test does not fail.