Should not have to build pebble-extension locally

[adamretter]

Instead of having to download and build pebble-extension first, we should ensure that pebble-extension is published via Yarn. We can then just depend on it by version number rather than needing the files locally (CC @ccheraa).

Once this is done we need to update .travis.yml and README.md with the reduced build steps.

[ccheraa]

FusionStudio Extension is now available on NPM

[adamretter]

@ccheraa Presumably that only happens for the last release. We should keep this open until we have nightly builds in-place then we can ensure that HEAD of fusion-studio is always testing against HEAD of fusion-studio-extension (more or less).