.travis.yml in formulaic-0.3.0 vs .travis.yml in formulaic-0.4.0

@@ -1,6 +1,7 @@
 rvm:
-  - "2.0"
-  - "2.1"
   - "2.2"
+  - "2.3"
+  - "2.4.0"
+  - "ruby-head"
 notifications:
   email: false