.travis.yml in rex-mime-0.1.1 vs .travis.yml in rex-mime-0.1.2

@@ -1,5 +1,9 @@
 sudo: false
+group: stable
+cache: bundler
 language: ruby
 rvm:
-  - 2.1.5
-before_install: gem install bundler -v 1.12.5
+  - 2.1
+  - 2.2
+  - 2.3.3
+  - 2.4.0