[EXPERIMENTAL WIP] Refactor fingerprint methods

[dazza-codes]

No review requested at this time.

• Experimental branch to play with fingerprints used to update the engine-cart app.
  • Changed fingerprint to use an MD5 instead of File.mtime
  • Might look into using https://github.com/ioquatix/fingerprint
• Applied some rubocop suggestions in some places.
• Figured out how to run specs locally, checking they also run on travis in this PR
  • discovered some weird spec failures because of rails version inconsistencies
  • clarified some of the destination values - removed some hard-coded values in specs