.travis.yml in yao-0.3.8 vs .travis.yml in yao-0.4.0

@@ -1,12 +1,15 @@
 language: ruby
 
 rvm:
-  - 2.2.7
-  - 2.3.4
-  - 2.4.1
+  - 2.3.7
+  - 2.4.4
+  - 2.5.1
   - ruby-head
 
+before_install:
+  - gem update --system --no-document
+  - gem i bundler --no-document
 install: bundle install --path vendor/bundle
 
 cache:
   directories: vendor/bundle