Correcting path of bower.json property "main"

The property `main` in bower.json is incorrect. It should be `dist/picturefill.js`.

This is necessary for some build tools. The specific build tool I was using when I found this was [wiredep](https://github.com/taptapship/wiredep).

In this case the browser was trying to load `bower_components/picturefill/picturefill.js` which does not exist.

Reference: [bower.json-spec#main](https://github.com/bower/bower.json-spec#main)