Ruby: add Gemfile with test-unit dependency to make test runner UI work in RubyMine/IntelliJ

When running the tests for the Ruby version inside IntelliJ (same seems
to apply to RuybMine) the test runner UI didn't work: The failure

"test framework quit unexpectedly"

was shown. Following the advice from

http://www.jetbrains.com/ruby/webhelp/test-unit-special-notes.html

to add the test-unit gem to the project fixes this issue.