Merge pull request #178 from activeadmin-plugins/travis_ruby_versions-1

bump ruby versions

Author: Fivell

.travis.yml

@@ -4,6 +4,7 @@ env:
     - RAILS=5.1.6.2
     - RAILS=5.2.2.1
 rvm:
-  - 2.3.8
   - 2.4.5
   - 2.5.5
+  - 2.6.5
+  - 2.7.0