travis: fix travis' problem with /dev/shm

See travis-ci/travis-cookbooks/issues#155
travis: don't require pysnmp, it is downloaded by setuptools
setup: be compatible with Python2.6 and provide check_output

We could also have used backquotes, but in the future, we also want
Python3.3 compatibility. Let's put a clean replacement function for
`check_output()`.