Adding "sudo: false" to allow faster builds on Travis.

[dhermes]

See
http://blog.travis-ci.com/2014-12-17-faster-builds-with-container-based-infrastructure/
for more info.

I will try to rework the gcloud-python-wheels stuff to utilize caching, but at the very least, this gives us better performance. Some snippets from the post:

Rather than wait for builds to start for a long time, your builds now start in less than 10 seconds.

and

The CPU resources are now guaranteed, which means less impact by noisy neighbors on the
same host machine. Build times throughout the day should be much more consistent on the
new container stack