Travis: Make JRuby produce test coverage info

[olleolleolle]

Summary

This PR enables test coverage data in JRuby using the environment variable JRUBY_OPTS=--debug.

Details

Travis output snippet before:

Finished in 8.04 seconds (files took 3.62 seconds to load)
346 examples, 0 failures

Coverage report generated for RSpec to
  /home/travis/build/cucumber/cucumber-ruby-core/coverage.
  0 / 4077 LOC (0.0%) covered.

Travis output snippet after:

Finished in 8.37 seconds (files took 5.94 seconds to load)
346 examples, 0 failures

Coverage report generated for RSpec to
  /home/travis/build/cucumber/cucumber-ruby-core/coverage.
  4048 / 4184 LOC (96.75%) covered.

(Bonus: We also pass travis-lint by adding the language key in the YAML configuration.)

Example coveralls page after this was merged: https://coveralls.io/jobs/26875981

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