Source maps broken in firefox

[glenjamin]

This seems related to browserify/browserify#681

If i remove the leading "/" from https://github.com/ForbesLindesay/browserify-middleware/blob/master/lib/compile.js#L40 then it works.

It looks like the "/" is there for a chrome issue though, so unsure if this is a neat fix.

In a related note, there doesn't seem to be any way to tweak the source map handling from calling code - so I can't see a way to change this behaviour from my side without an upstream change.

[ForbesLindesay]

We no longer to any munging of source maps, so any remaining bugs should be directed to browserify instead.