First tox run fails

With a brand-new checkout, running `tox -e py27` fails with

``` python
======================================================================
ERROR: testGeneration (apitools.gen.client_generation_test.ClientGenerationTest)
----------------------------------------------------------------------
Traceback (most recent call last):
File ".../apitools/apitools/gen/client_generation_test.py", line 61, in testGeneration
retcode = subprocess.call(args)
File "/usr/lib/python2.7/subprocess.py", line 522, in call
return Popen(*popenargs, **kwargs).wait()
File "/usr/lib/python2.7/subprocess.py", line 710, in __init__
errread, errwrite)
File "/usr/lib/python2.7/subprocess.py", line 1327, in _execute_child
raise child_exception
OSError: [Errno 2] No such file or directory
-------------------- >> begin captured logging << --------------------
root: INFO: Testing API drive.v2 with command line: gen_client --client_id=12345 --client_secret=67890 --discovery_url=drive.v2 --outdir=generated --overwrite client
--------------------- >> end captured logging << ---------------------
```