Update/add elm 0.15 binaries in elm-travis-cache

[chendrix]

How difficult would it be to add/update the 0.15 binaries to elm-travis-cache?

Or, alternatively, are there setup instructions to be able to generate our own binaries that can be wget in a travis build?

[jvoigtlaender]

Seen the npm elm installer? This might be a way to pull in the binaries in a travis build. Alternatively, the installer's repo contains 0.15 binaries (in the vendor directory) that you might be able to use.

[chendrix]

That worked quite nicely

On Travis it builds extremely quickly now.

You can see how I've used it in my .travis.yml