Update supported ruby versions in travisCI config file

Author: davydovanton

.travis.yml

@@ -3,9 +3,10 @@ sudo: false
 cache: bundler
 before_install: gem install bundler -v 1.11.2
 rvm:
-  - 2.2.5
   - 2.3.1
   - 2.4.0
+  - 2.5.0
+  - 2.6.0
   - jruby-head
   - rbx-2
   - ruby-head