[CMSP-223] WP Redis housekeeping

[jazzsequence]

• removes NPM files and Grunt - we are not using the one thing Grunt was being called for which was a wp readme to markdown script.
• adds valid license to composer file
• This PR also updates develop to match master by bumping the relevant version numbers

Since this is not affecting any files in the actual plugin, there's no reason to put this in a release.

[jspellman814]

@jazzsequence Can we update this line too (remove package.json mention): https://github.com/pantheon-systems/wp-redis/blob/master/CONTRIBUTING.md?plain=1#L29?