Release 2.4.0

[JoshCheek]

Waiting on the ruby gems fix to be released so that the tests pass in Travis. Once it's released

• [x] Wait for Rubygems > 2.7.1 to be released
• [x] Rerun Travis CI to make sure it passes (manually upgrade the version of ruby gems if necessary)
• [x] Fix https://github.com/JoshCheek/seeing_is_believing/issues/122
• [x] Bump the version to 2.4.0
• [x] Release it
• [x] Update the Atom plugin (here)
• [x] Figure out how to publish the Atom plugin (here)
• [x] At that point, users can upgrade their SiB version and then add the --local-cwd flag to the invocation, so close:
  • [x] https://github.com/JoshCheek/atom-seeing-is-believing/issues/18