[WIP] Use pip-accel for building jenkins workers

@benpatterson @fredsmith FYI
No, you need more than just the .boto file. You need to have boto installed in the virtualenv for pip-accel to take advantage of the s3 cache, and the maintainer of pip-accel didn't want to include boto as a requirement. So you either need to install it yourself, or you need to install pip-accel like this: `pip install pip-accel[s3]`