Test on ruby-head, allow failures

Author: sodabrew

.travis.yml

@@ -16,6 +16,7 @@ rvm:
   - 2.2
   - ree
   - rbx-2
+  - ruby-head
 matrix:
   include:
     - rvm: 2.0.0
@@ -30,3 +31,5 @@ matrix:
     - rvm: 2.0.0
       env: DB=mysql55
       os: osx
+  allow_failures:
+    - rvm: ruby-head