Allow ruby-head failures

Looks like Travis has very unreliable Mac OS X builds when using
`ruby-head`: Bundler is missing on the installation, and sometimes
setting up the interpreter itself times out.

Let's ignore it for now

Author: vmg

.travis.yml

@@ -20,6 +20,7 @@ matrix:
   fast_finish: true
   allow_failures:
     - rvm: rbx-2
+    - rvm: ruby-head
 
 install:
   - ./vendor/libgit2/script/install-deps-${TRAVIS_OS_NAME}.sh