.travis.yml in countries-2.0.5 vs .travis.yml in countries-2.0.6

@@ -1,13 +1,14 @@
 sudo: false
 cache: bundler
 language: ruby
 rvm:
- - 2.2.3
- - 2.3.1
+ - 2.2.6
+ - 2.3.3
+ - 2.4.0
  - ruby-head
 before_install:
   - gem update --system
-  - gem install bundler --pre
+  - gem install bundler
 matrix:
   allow_failures:
     - rvm: ruby-head