Force travis to use bundler 1.12.5

Workaround for cbeer/engine_cart#64

.travis.yml

@@ -17,5 +17,8 @@ notifications:
       - "irc.freenode.org#projecthydra"
     template:
       - "%{repository}//%{branch}@%{commit} by %{author}: %{message} - %{build_url}"
+before_install:
+  - gem uninstall -i /home/travis/.rvm/gems/ruby-2.3.1@global bundler -v 1.13.0
+  - gem install bundler -v 1.12.5
 before_script:
   - jdk_switcher use oraclejdk8