Adds support for ruby head (at 2.6 currently) to travis

.travis.yml

@@ -5,6 +5,7 @@ rvm:
   - 2.3
   - 2.4
   - 2.5
+  - ruby-head
 
 before_install:
   - gem install bundler
@@ -19,6 +20,13 @@ gemfile:
   - gemfiles/mongoid_5.0.gemfile
   - gemfiles/mongoid_6.0.gemfile
 
+matrix:
+  include:
+    - rvm: ruby-head
+      env: RUBYOPT="--enable-frozen-string-literal"
+  # allow_failures:
+  #   - rvm: ruby-head
+
 addons:
   code_climate:
     repo_token: b84a33c35c698270ad54261655bc25161a0853386825a2d54fb1c7a11c2b2785