Update bundler and bin/bundle

[mitchellhenke]

Relevant Ticket or Conversation:

Somewhat related to #739, but primarily to support the changes in https://github.com/18F/identity-devops/pull/7078

Description of Changes:

This PR updates the bundler version and adds the updated binstub for bundler. It has been tested in my sandbox.

PR Checklist:

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2. Have you tagged the appropriate dev(s) for review?
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