Add ruby-head to Travis as allow_failures.

* allow_failures is because it's good to know new version Ruby's issue
  as faster before the release.
* fast_finish is to get the Travis result as faster
  without waiting the result of the "allow_failures" items.
  See https://blog.travis-ci.com/2013-11-27-fast-finishing-builds/

.travis.yml

@@ -8,6 +8,7 @@ rvm:
   - 2.2.6
   - 2.3.3
   - 2.4.1
+  - ruby-head
   - jruby-19mode
   - jruby-9.1.8.0
 git:
@@ -27,3 +28,5 @@ matrix:
     - rvm: 2.4.1
       gemfile: gemfiles/alaska
       env: ALASKA=1
+    - rvm: ruby-head
+  fast_finish: true