Removed /opt/boxen/homebrew/share/python from PATH to fix brew doctor warning:

Warning: /opt/boxen/homebrew/share/python is not needed in PATH.
Formerly homebrew put Python scripts you installed via `pip` or `pip3`
(or `easy_install`) into that directory above but now it can be removed
from your PATH variable.
Python scripts will now install into /opt/boxen/homebrew/bin.
You can delete anything, except 'Extras', from the /opt/boxen/homebrew/share/python
(and /opt/boxen/homebrew/share/python3) dir and install affected
Python packages anew with `pip install --upgrade`.