Run specs on ruby-head.

Author: gkellogg

.travis.yml

@@ -7,10 +7,12 @@ rvm:
   - 2.5
   - 2.6
   - 2.7
+  - ruby-head
   - jruby
 cache: bundler
 sudo: false
 matrix:
   allow_failures:
     - rvm: jruby
+    - rvm: ruby-head
 dist: trusty