.travis.yml in terraforming-0.12.0 vs .travis.yml in terraforming-0.13.0

@@ -1,8 +1,9 @@
 language: ruby
 rvm:
-- 2.3.1
-- 2.2.5
+- 2.4.0
+- 2.3.3
+- 2.2.6
 - 2.1.10
 env:
   global:
     - secure: n0mxwnyjuXI4mJO4mp++2TnsPJ+XgCF/J1U2L5piE5j3xMhSU+5V0JrA1uFlS0Pemb44M7BjgmF9S4G35BwyAQpctpCYhqy9tFa6+Y6nxEv5hCv2cZz7BSAZM6eb+zq20409hxTHRaQOr1DBeE4R5S2PrmOXRqvYfTRv3LNSLFk=
   matrix: