"make -k develop" fails occasionally

[hannes-ucsc]

pip doesn't seem to correctly serialize access to the virtualenv.

http://jenkins.cgcloud.info/job/cgcloud/341/consoleText

Installing cwutil script to /mnt/ephemeral/workspace/cgcloud/venv/bin
Installing cwutil script to /mnt/ephemeral/workspace/cgcloud/venv/bin
error: [Errno 2] No such file or directory: '/mnt/ephemeral/workspace/cgcloud/venv/bin/cwutil'
Installing sdbadmin script to /mnt/ephemeral/workspace/cgcloud/venv/bin
cd mesos && python2.7 -m nose --verbose
cd toil && python2.7 setup.py egg_info develop