Fix the sourceMap url on minified bundles

Summary:
public

The minified bundle was always being generated with the `bundle.js` as the source map url.

Reviewed By: martinbigio

Differential Revision: D2773725

fb-gh-sync-id: 02cda95eb172fc373ce4e605418cbfcec22f433d

react-packager/src/Bundler/Bundle.js

@@ -188,7 +188,7 @@ class Bundle {
       const minifyActivity = Activity.startEvent('minify');
       this._minifiedSourceAndMap = UglifyJS.minify(source, {
         fromString: true,
-        outSourceMap: 'bundle.js',
+        outSourceMap: this._sourceMapUrl,
         inSourceMap: this.getSourceMap(),
         output: {ascii_only: true},
       });