Fix travis not using the right version of bundler

Author: evheny0

.travis.yml

@@ -1,7 +1,9 @@
 language: ruby
 sudo: false
 cache: bundler
-before_install: gem install bundler -v 1.11.2
+before_install:
+  - gem install bundler -v 1.11.2
+  - gem update --system
 rvm:
   - 2.3.1
   - 2.4.0