Add -p switch to runner.rb for running UI tests in parallel

Usage: `./runner.rb -p 3` will run UI tests 3 at a time.
Switch from threads to processes so ENV vars aren't shared
Merge branch 'parallel-ui-tests' into staging

Conflicts:
	dashboard/test/ui/runner.rb
Merge branch 'staging' of https://github.com/code-dot-org/code-dot-org into staging