Prepare for "mapbox-gl-style-spec" package changes

[lucaswoj]

Hello!

I work on at Mapbox on Mapbox GL. We are preparing to make some changes to the mapbox-gl-style-spec package which may affect you:

• the mapbox-gl-style-spec package will be deprecated and replaced by @mapbox/mapbox-gl-style-spec
• mapbox-gl-style-spec package will begin using es6 features, requiring the use of transpilation (if you're using browserify, this'll happen automatically via bubleify)
• the file structure of mapbox-gl-style-spec will change (which will affect the use of require(mapbox-gl-style-spec/lib/**/*.js))

You can read more about these changes at mapbox/mapbox-gl-js#3946 and mapbox/mapbox-gl-js#3656.

Let me know if you have any questions or need any help!