Make the Makefile work on OSX

Apparently, `which -a` on OSX likes to print the name twice, which breaks the
minimization/cat phase.

Also, I ran into an issue where stumbler.zip was non-existent, and thus rm was
failing, stopping the Makefile for some reason. Adding `-f` fixes that.
Merge pull request #8 from padenot/master

Make the Makefile work on OSX