Fix incorrect main field in npm package

[robinnorth]

No description provided.

[BehindTheMath]

This was actually on my to do list.

However, I think the main field should point to index.js. If I'm not mistaken, only Node uses the main field, to determine the entry point of the module. Anyone requireing this package in Node is doing the bundling themselves, so we might as well serve the source itself without the Browserify wrapper code. Someone who is using the library in a broswer in a script tag (using unpkg.com) will link to pjax.js directly.

[robinnorth]

You are indeed right, it really should be the other way around. Clearly it used to point to the built library file, but it does make more sense to point at the source. I'll amend my PR for you.