[travis ci] Lock bundler version

Author: zetachang

.travis.yml

@@ -29,7 +29,7 @@ before_install:
   - "if [ $(phantomjs --version) != $PHANTOMJS_VERSION ]; then wget https://github.com/Medium/phantomjs/releases/download/v$PHANTOMJS_VERSION/phantomjs-$PHANTOMJS_VERSION-linux-x86_64.tar.bz2 -O $PWD/travis_phantomjs/phantomjs-$PHANTOMJS_VERSION-linux-x86_64.tar.bz2; fi"
   - "if [ $(phantomjs --version) != $PHANTOMJS_VERSION ]; then tar -xvf $PWD/travis_phantomjs/phantomjs-$PHANTOMJS_VERSION-linux-x86_64.tar.bz2 -C $PWD/travis_phantomjs; fi"
   - "phantomjs --version"
-  - gem install bundler
+  - gem install bundler -v 1.13.7
 script:
   - bundle exec rake test_app
   - bundle exec rake