Weather: don't force specific grep location in bsd
Weather: don't force specific grep location in bsd

It is possible that grep can be installed in any locations, 
so we should not just assume '/user/local/bin' on BSD machines.
We must rely on the user reading the manual and updating grep.

tacahiroy/tmux-powerline,tacahiroy/tmux-powerline,tacahiroy/tmux-powerline