Minor fix

Append sourceMappingUrl in minified files

This fix makes the all JS files in the dist folder have the source mapping file pointed out so that bundlers can generate a correct final map file for the application or library that bundles dexie into it.

If dexie was used without bundling it into the app, this change will have no benefit as chrome devtools is still able to locate the map files without the mapfile comment-line.