Localforage, browserify and IE8 do not play nicely together

When requiring localforage with browserify IE8 baulks with 'Object doesn't support this property or method' type errors. 

This is due to the fact that browserify will bundle up the `promise` module which is not es3 compatible. The compiled version (in the dist directory) uses a different promise library - `es6-promise` which _is_ es3 compatible. This PR contains a tweak so that browserify will also bring in `es6-promise`.

It's not a very elegant solution, in fact it feels a bit hacky, but I erred on the side of caution and tried not to touch anything.

Note that even with this change a browserified version of this package is still not quite IE8 ready. For that you need to filter out the es3 reserved keywords using something like [dekeywordify](https://github.com/pluma/dekeywordify)
Probably - that would be much nicer. You then wouldn't need the changes to the browser field in package.json either. I was worried that it'll mess up some sort of requirejs type thing that was relying on `require('promise')` being explicitly there as 'promise', or some node use case where people were relying on the other promise dependency (is this even possible?), so opted for safety over niceness.