Fix sourcemaps in JSXTransformer for Firefox

Fixes #1725
> Firefox has problems when the sourcemap source is a proper URL with a protocol and hostname, so just use the path.

I'm assuming you meant the next one.

> Things seem to work fine if we just use filename.js instead of path/to/filename.js, but ...

So given `http://hostname:port/path/to/filename.js`, we have to strip it down to `path/to/filename.js` at least. We could strip it all the way to `filename.js`, which also works (and is what I originally tested). The potential downside (I didn't back this up with a test) is that there may be `path/to/filename.js` and `otherpath/to/filename.js` on the same page and I thought perhaps the devtools would get confused mapping them back from the sourcemap if we just used `filename.js`