Fix startup issues when pygments is already detected

This uses the system version of pyments over the bundled version and fixes the errors when starting Anki such as

     /home/user/Anki/addons/code_highlight_addon/pygments/plugin.py:39:        
     UserWarning: Module pygments was already imported from /home/user/Anki/addons/code_highlight_addon/pygments/__init__.pyc, but /usr/lib/python2.7/dist-packages is being added to sys.path
     import pkg_resources