Merge pull request #327 from cben/ruby-2.6

Start testing ruby 2.6

Author: Mooli Tayer

.travis.yml

@@ -4,6 +4,7 @@ rvm:
   - "2.3.0"
   - "2.4.0"
   - "2.5.0"
+  - "2.6.0-preview1"
 sudo: false
 cache: bundler
 script: bundle exec rake $TASK