Switch from which to command -v

So that you don't have to redirect sdterr to /dev/null to not get output like the following:

    $ n 0.8.25
    which: no wget in (/usr/local/bin:/usr/bin:/bin:/home/eivind/bin:/usr/bin/core_perl:/home/eivind/.gem/bin:/home/eivind/node_modules/.bin)