Allow failures for Rubinius build

The build with Rubinius spuriously fails with the following error:

1) Overcommit::HookContext::CommitMsg#commit_message_lines strips
   comments and trailing diff
   Failure/Error: subject { context.commit_message_lines }
   Errno::ENOENT:
   No such file or directory - /tmp/commit-message20150420-4267-14fo3is
   kernel/common/io.rb:942:in `sysopen'
   kernel/common/file.rb:1215:in `initialize'
   kernel/alpha.rb:94:in `new'
   kernel/common/io.rb:624:in `open'
   kernel/common/io.rb:431:in `foreach'
   kernel/common/io.rb:448:in `readlines'
   ./lib/overcommit/hook_context/commit_msg.rb:29:in `raw_commit_message_lines'
   ./lib/overcommit/hook_context/commit_msg.rb:17:in `commit_message_lines'
   ./spec/overcommit/hook_context/commit_msg_spec.rb:37:in `__script__'
   kernel/common/hash.rb:356:in `fetch'
   ./spec/overcommit/hook_context/commit_msg_spec.rb:40:in `__script__'
   kernel/common/eval.rb:101:in `instance_exec'
   kernel/bootstrap/array.rb:97:in `map'
   kernel/bootstrap/array.rb:97:in `map'
   kernel/bootstrap/array.rb:97:in `map'
   kernel/common/kernel.rb:497:in `load'
   kernel/delta/code_loader.rb:66:in `load_script'
   kernel/delta/code_loader.rb:152:in `load_script'
   kernel/loader.rb:655:in `script'
   kernel/loader.rb:809:in `main'

Until we have time to figure this out, ignore Rubinius failures since to
my knowledge it does not have a huge number of adopters who are also
using it for Overcommit.

Author: sds

.travis.yml

@@ -12,6 +12,10 @@ rvm:
   - jruby-19mode
   - rbx-2
 
+matrix:
+  allow_failures:
+    - rvm: rbx-2
+
 before_script:
   - git config --global user.email "travis@travis.ci"
   - git config --global user.name "Travis CI"