Update travis setup

Author: Daniel Nitsikopoulos

.travis.yml

@@ -2,7 +2,7 @@
 script: bundle exec rspec spec
 language: ruby
 sudo: false
-before_install: gem update bundler
+before_install: gem install bundler:1.17.2
 rvm:
   - 2.3.0
   - 2.4.0