Update to the latest bundler version

There was an issue where Travis build was erring because the bundler
version was 1.2.3, and we explicitly upgrade rubygems via travis.yml.
Thus, the result was that rubygems 2.0 is no longer compatible with
older bundler versions. Thus let's always grab the latest version of
bundler for Travis builds.

.travis.yml

@@ -4,6 +4,7 @@ rvm:
   - 1.9.3
 before_install:
   - gem update --system
+  - gem update bundler
 before_script:
   - "bundle exec rake db:migrate"
 gemfile: