Add ruby-head to build

Build `ruby-head` and allow failures.

Author: Tom Johnson

.travis.yml

@@ -1,19 +1,24 @@
 language: ruby
 bundler_args: --without debug
 script: "bundle exec rspec spec"
+cache: bundler
+sudo: false
+dist: trusty
+
 env:
   - CI=true RACK_VERSION=1.6
   - CI=true RACK_VERSION=2.0
+
 rvm:
   - 2.2
   - 2.3
   - 2.4
+  - ruby-head
   - jruby-9
   - rbx-3
-cache: bundler
-sudo: false
+
 matrix:
   allow_failures:
     - rvm: jruby-9
     - rvm: rbx-3
-dist: trusty
+    - rvm: ruby-head