Use this.sourceMap instead of query.sourceMap

Webpack already [gives us this.sourceMap](https://github.com/petehunt/jsx-loader/pull/19#issuecomment-53692088), so we should use it.
It will be `false` if user doesn't emit source maps in config.
Merge pull request #25 from gaearon/master

Use this.sourceMap instead of query.sourceMap