Add truffleruby-head to the CI

Author: gogainda

Diff for: .travis.yml

@@ -7,6 +7,13 @@ notifications:
     # https://coveralls.zendesk.com/hc/en-us/articles/203484329-Parallel-Build-Webhook
     - secure: "YnHYbTq51ySistjvOxsuNhyg4GLuUffEJstTYeGYXiBF7HG5h43IVYo8KNuLzwkgsOYBcNo+YMdQX7qCqJffSbhsr1FZRSzBmjFFxcyD4hu+ukM2theZ4mePVAZiePscYvQPRNY4hIb4d3egStJEytkalDhB3sOebF57tIaCssg="
 rvm:
+  - 2.2.10
+  - 2.3.8
+  - 2.4.9
+  - 2.5.7
+  - 2.6.5
+  - 2.7.0
+  - ruby-head
   - truffleruby-head
 gemfile:
   - gemfiles/rails42.gemfile