Fix the flag '--version' not working when the folder 'xwalk_app_template' exists

The script won't download the build tool when the folder 'xwalk_app_template' exists.
After this fix, we can switch between versions more easily.

Solution:
1: If '--version' specified for make_webapp.py
  1) If we have previously downloaded the version,
      Replace xwalk_app_template to the version
  2) If not downloaded, download it
2: If no '--version' specified, use previous one
BUG=#24