Dockerfile: add step "gem install bundler"

[DeeDeeG]

Context

Trying to fix poltergeist timeout issues discussed in #457 and #455

Summary of Changes

• Adds a step to the Dockerfile to do gem install bundler. This will get us the latest bundler version. As opposed to using the bundler version included in RVM's pre-built Ruby 2.3.1 image.
  • At the moment, this change means we will go from using bundler 1.13.6 to -> 1.16.1, but we would continue to receive the latest bundler version on an ongoing basis.

Checklist

• Tested Mobile Responsiveness
• Added Unit Tests
• CI Passes
• Deploys to Heroku on test Correctly (Maintainers will handle)
• Added Documentation (Service and Code when required)