Fixed update.json creation for the first run

``` bash
git clone https://github.com/gae-init/gae-init.git
./run.py

Traceback (most recent call last):
  File "./run.py", line 422, in <module>
    run()
  File "./run.py", line 399, in run
    check_for_update()
  File "./run.py", line 264, in check_for_update
    update_json = open(FILE_UPDATE, 'w')
IOError: [Errno 2] No such file or directory: 'temp/update.json'
```
oops