Fix fetching sourcemap in genymotion. Fixes #5338

Summary:Source maps are broken on Genymotion right now as they aren't being loaded from the correct URL. refer - https://github.com/facebook/react-native/issues/5338#issuecomment-188232402

**Test plan**

Build and install UIExplorer from master branch in genymotion and enable hot reload. When you change a file and save it, you'll see a Yellow box due to source map fetching failed, as per the referenced comment.

Doing the same for this branch doesn't produce any yellow boxes.
Closes https://github.com/facebook/react-native/pull/6594

Differential Revision: D3088218

Pulled By: martinbigio

fb-gh-sync-id: 0d1c19cc263de5c6c62061c399eef33fa4ac4a7b
shipit-source-id: 0d1c19cc263de5c6c62061c399eef33fa4ac4a7b

packager/react-packager/src/Bundler/index.js

@@ -170,9 +170,9 @@ class Bundler {
     });
   }
 
-  _sourceHMRURL(platform, host, port, path) {
+  _sourceHMRURL(platform, path) {
     return this._hmrURL(
-      `http://${host}:${port}`,
+      '',
       platform,
       'bundle',
       path,
@@ -217,7 +217,7 @@ class Bundler {
     return this._bundle({
       ...options,
       bundle: new HMRBundle({
-        sourceURLFn: this._sourceHMRURL.bind(this, options.platform, host, port),
+        sourceURLFn: this._sourceHMRURL.bind(this, options.platform),
         sourceMappingURLFn: this._sourceMappingHMRURL.bind(
           this,
           options.platform,