.travis.yml in prorate-0.5.0 vs .travis.yml in prorate-0.6.0

@@ -1,8 +1,11 @@
 rvm:
-- 2.2.5
-- 2.3.3
-- 2.4.1
+- 2.2
+- 2.3
+- 2.4
+- 2.5
+- 2.6
+- 2.7
 
 services:
 - redis
 
 dist: trusty # https://docs.travis-ci.com/user/trusty-ci-environment/