gnome-shell-extension-tool: Fix error after creating extension

Commit 7a8a00c705 cleaned up the code to move all
files to a dictionary, which accidentally left an undefined "extensionjs_path"
error. Fix that error.

https://bugzilla.gnome.org/show_bug.cgi?id=661623

src/gnome-shell-extension-tool.in

@@ -148,5 +148,6 @@ use an extension title clicktofocus@janedoe.example.com.'''
         f.close()
 
     print "Created extension in %r" % (extension_path, )
+    extensionjs_path = os.path.join(extension_path, 'extension.js')
     subprocess.Popen(['gnome-open', extensionjs_path])
     sys.exit(0)