Switch to using which instead of command -s
Switch to using which instead of command -s

`command -s` is only available in 2.2.0, so using `which` instead of `command -s` would make Pure compatible with most folks that have not upgraded to 2.2.0 for one reason or another.

No reason to keep lingering in 2.1.2, but this change adds support to fish 2.1.2 and possibly lower.
rafaelrinaldi/pure