DEV: Introduce parallel rspec testing (PR #7296)

Adds the parallel_tests gem, and redis/postgres configuration for running rspec tests in parallel. To use:

rake parallel:rake[db:create]
rake parallel:rake[db:migrate]
rake parallel:spec

This brings the test suite from 12m20s to 3m11s on my macOS machine


You’ve signed the CLA, davidtaylorhq. Thank you! This pull request is ready for review.

Looks cool, I’m gonna try it out!