Avoid error from aliased `which` during install

The same fix as #207, just for fools like me who have `which` aliased to `type` in OS X. Without this escape, a very cryptic error message is emitted (though the install ultimately succeeds, as long as one actually has `curl` installed).
Merge pull request #274 from evocateur/patch-1

Avoid error from aliased `which` during install