Remove browserify-shim dependency

[jefbarn]

There's no need to force browserify (browserify-shim specifically) in the dependencies. Causes annoying errors and warnings for projects using alternative bundlers (webpack) and polyfills (core-js).

[gmaclennan]

This exists so that the packaged version does not include the Leaflet code. I would welcome any PR that makes this work with other bundlers. I don't use them so I would not know where to start.

[mchlroy]

I'm having the same problem. I have warning telling me to install browserify >= 2.3 because browserify-shim depends on it.

Correct me if I'm wrong, but I shouldn't need to have browserify-shim installed at all if I'm simply using your package along with leaflet, can't it simply be moved into the devDependencies? Even the browserify-shim README states to install it in devDependencies.