add most recent ruby versions

vroldanbet · vroldanbet · commit 10a44d5ad1ac · 2019-11-14T12:03:49.000+01:00
diff --git a/.travis.yml b/.travis.yml
@@ -3,6 +3,10 @@ rvm:
   - 2.0.0
   - 2.1
   - 2.2
+  - 2.3
+  - 2.4
+  - 2.5
+  - 2.6
   # optional
   - ruby-head
   - jruby-19mode
