Fix mapbox-gl peerDependencies specification

[jseppi]

I think the way that the mapbox-gl peerDependency was specified in #280 was incorrect, as I am still getting an error during npm install:

$ npm i --save mapbox-gl@latest
npm WARN @mapbox/[email protected] requires a peer of mapbox-gl@^0.47.0 but none is installed. You must install peer dependencies yourself.

Here is what the package.json docs say:

Thus, if you’ve worked with every 1.x version of the host package, use "^1.0" or "1.x" to express this. If you depend on features introduced in 1.5.2, use ">= 1.5.2 < 2".

This PR changes the mapbox-gl peerDependency to match the format described by the above docs.