Fix Travis CI build.

Allow JRuby to fail, since it is crashing after all specs pass.

example: https://travis-ci.org/simi/generator_spec/jobs/42043223

It is caused by https://github.com/stevehodgkiss/generator_spec/blob/master/spec/generator_spec/generator_example_group_spec.rb.

I've tried to reuse [rspec-rails shared_example.rb](https://github.com/rspec/rspec-rails/blob/master/spec/support/shared_examples.rb), but that erors remains.