Update install_deps

Looks like the appengine sdk has had a version update.
Running install_deps gives:
```python
Downloading the AppEngine SDK...
Traceback (most recent call last):
  File "./install_deps", line 37, in <module>
    zipfile = ZipFile(StringIO(url.read()))
  File "/usr/lib/python2.7/zipfile.py", line 714, in __init__
    self._GetContents()
  File "/usr/lib/python2.7/zipfile.py", line 748, in _GetContents
    self._RealGetContents()
  File "/usr/lib/python2.7/zipfile.py", line 763, in _RealGetContents
    raise BadZipfile, "File is not a zip file"
zipfile.BadZipfile: File is not a zip file
```

Not a very permanent fix but it works for now. If I get time later in the week I can look into finding a stable link. Otherwise if if anyone else wants to have a go feel free.