.travis.yml in formulaic-0.4.0 vs .travis.yml in formulaic-0.4.1

@@ -1,7 +1,8 @@
 rvm:
-  - "2.2"
   - "2.3"
-  - "2.4.0"
+  - "2.4"
+  - "2.5"
+  - "2.6"
   - "ruby-head"
 notifications:
   email: false