Create development version of Parity forumla
Create development version of Parity forumla

This change adds a `devel` version of the Parity forumla that can be
used to test the packaged executables without pushing a new version
prematurely to users.

Documentation:
https://github.com/Homebrew/homebrew/blob/master/share/doc/homebrew/Formula-Cookbook.md#unstable-versions-devel-head

thoughtbot/homebrew-formulae