Fix TravisCI bundler version
Fix TravisCI bundler version

The TravisCI version of bundler is out of date, which can cause some problems
install dependencies. Updating bundler as part of the install setup is the
fix recommended by Travis (See travis-ci/travis-ci#3531)

More info:
https://github.com/rubygems/rubygems/issues/1419

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet