Document how to cache Sauce Connect binary

[vweevers]

The following seems to do the trick on Travis, because it tells sauce-connect-launcher to download the Sauce Connect binary in a npm postinstall step. And the binary is stored inside node_modules, so combined with Travis' node_modules cache (which is enabled by default) the binary is only ever downloaded once.

# .travis.yml
env:
  - SAUCE_CONNECT_DOWNLOAD_ON_INSTALL=true