Add ruby-head and jruby-head to Travis, but allow them to fail

This way we'll know if/how the build is failing on the latest versions, but
without marking the entire build as a failure if only the head versions fail.

See:
  - https://github.com/rspec/rspec-core/blob/master/.travis.yml
  - http://blog.travis-ci.com/2013-11-27-fast-finishing-builds/

.travis.yml

@@ -1,6 +1,7 @@
 language: ruby
 cache: bundler
 rvm:
+  - ruby-head
   - 2.1.2
   - 2.1.1
   - 2.1.0
@@ -9,6 +10,12 @@ rvm:
   - 1.9.2
   - 1.8.7
   - ree
-  - jruby-18mode
+  - jruby-head
   - jruby-19mode
+  - jruby-18mode
   - rbx-2
+matrix:
+  allow_failures:
+    - rvm: ruby-head
+    - rvm: jruby-head
+  fast_finish: true