Update bundler before installing gems

The default bundler included in Travis CI Ruby environments is too old
and contains a bug that triggers a failure and prevents testing.

Refs: rubygems/bundler#3558 travis-ci/travis-ci#3531

.travis.yml

@@ -8,5 +8,6 @@ rvm:
 script: bundle exec rake
 before_install:
   - gem update --system
+  - gem install bundler
 services:
   - redis-server