Update bundler before running tests on Travis CI

See rubygems/bundler#3558

Author: daronco

.travis.yml

@@ -2,6 +2,8 @@ sudo: false
 language: ruby
 services:
   - redis-server
+before_install:
+  - gem install bundler
 bundler_args: "--without production development"
 before_script:
   - "cp spec/rails_app/config/database.yml.travis spec/rails_app/config/database.yml"