Fix issue with underscores in package names. Fixes #91

Before this commit, `python_pip` reinstalled packages that contain underscores in their names.
More details at https://github.com/poise/python/issues/91


PS.: I ran into some issues with the tests and VM memory size. In my local development I increased memory size to 1030MB and it worked fine. The error I got was `FATAL: Chef::Exceptions::ChildConvergeError: Chef run process terminated by signal 9 (KILL)`, that I did some research and it is related to not enough memory. I still don't know why that happened.