Fix which command in zsh (#604).

[dhcmrlchtdj]

use command which to ignore zsh's buildin command which.

zsh's which will not return the path to node if someone create a alias like alias node="node --harmony".

[ljharb]

Thanks!

This works well in all tested shells, but fails the nvm_ls_current tests in zsh only.

If you can both a) add a test that would fail without this change, and b) make that failing test pass both with and without this change, then I'm happy to merge this.

[dhcmrlchtdj]

er, I need some help.
It can be test by the follow command, but I'm not familiar with shell script.

$ alias node="node --harmony" && nvm use v0.11.14 &> /dev/null && nvm current
v0.11.14 # pass
system # failure

nvm use doesn't work as I expected in script.

[ljharb]

You got the existing test passing, and I think i can modify the nvm_ls_current tests to test for this pretty easily. Thanks, I'll merge this now.