Move pattern libraries to HTTP/2

[james-nash]

Is your feature request related to a problem? Please describe.
We serve our various pattern library environments via unencrypted HTTP. It would be nice to serve them via HTTP/2, as we do for the Buildit website.

Aside from that warm fuzzy feeling you get from making life difficult for governments trying to spy on their citizens' web browsing, there are some practical benefits too:

• HTTP/2 should be faster than ye olde HTTP thanks to header compression, pipelining and other goodness
• Some recent JS APIs - e.g. service workers - aren't available when served over HTTP. While we don't currently make use of those in Gravity, it's conceivable that we may want to in the future.

Describe the solution you'd like
Mimick the setup we use for the Buildit website. From what (little) I understand, that amounts to:

• Setting up Cloudfront CDN
• Ensuring http URLs will permanently redirect to the corresponding https URLs
• Adding some cache invalidation to our .travis.yml, so that new releases clear the CDN cache as needed.