The Travis docs builds were broken for two reasons. First, we were using
a function not available on Python 2.6 (subprocess.check_output), and
second, we were assuming a full Git tree with a master branch (needed
for linking code).

The check_output call has been replaced with a more direct Popen
request, and the build file now explicitly fetches the master branch.

I reproduced the problems locally, and this fixed them both. I haven't tested

on Travis yet.