[CI] Update Bundler to 1.11.2 on Travis

Latest builds on Travis are broken because of bundler is too old and contains some issues

See: rubygems/bundler#3558, travis-ci/travis-ci#3531

Related: #538

.travis.yml

@@ -29,7 +29,7 @@ before_script:
 script:
   - SERVER=launch TEST_CLUSTER_COMMAND=/usr/share/elasticsearch/bin/elasticsearch TEST_CLUSTER_PARAMS='-Des.default.path.conf=/etc/elasticsearch/ -Des.default.path.logs==/var/log/elasticsearch/' bundle exec rake test:all
 
-install: "gem uninstall bundler -axI && gem install bundler -v 1.6.2"
+install: "gem uninstall bundler -axI && gem install bundler -v 1.11.2"
 
 notifications:
   disable: true