Use bundler 1.7 on travis

.travis.yml

@@ -3,6 +3,7 @@ sudo: false
 script: 'ci/travis.rb'
 before_install:
   - "rvm current | grep 'jruby' && export AR_JDBC=true || echo"
+  - gem install bundler
 before_script:
   - bundle update
 cache: bundler