Warn if pip-check-reqs is running from another environment

[MrMino]

It is a pretty common pitfall to run a globally installed command that was reinstalled within the active virtualenv. Right after local virtualenv installation, the shell still resolves installed command to the global one, until user enters hash -d (or rehash in ZSH).

Since this will most likely make pip-check-reqs fail, we should warn when this happens.
Extra points for making the warning text yellow when stderr is a TTY.

See also #82.