Don't fingerprint assets in development (dev perf fix) (#1018)

no issue
- doing so causes a double pass through asset-rewrite and forces both to take much longer. This cuts 3s off of rebuilds.  Using asset-rewrite as is done in dev mode is also suspicious, likely a better setup could be devised that doesnt require rewrite to interop with the local ghost server

ghost/admin/ember-cli-build.js

@@ -92,7 +92,7 @@ module.exports = function (defaults) {
             }
         },
         fingerprint: {
-            enabled: true,
+            enabled: isProduction,
             extensions: ['js', 'css', 'png', 'jpg', 'jpeg', 'gif', 'map']
         },
         minifyJS: {