Changed which... > /dev/null to which -s ...

OS X 10.10 seems to have done something causing which tmux > /dev/null to return false

Which does however have an option for not outputting what it found but return 0 or 1 depending on if it was found, achieving the same goal.
Changed which -s to type

Linux doesn't seem to have -s with the which binary.
Merge pull request #236 from thorerik/patch-1

Change from using which to type.