dotfiles.sh: Replace `which` with `command -v`

dotfiles.sh: Replace `which` with `command -v`

I learned (via the Git list [1]) that `which` is not part of POSIX.
The POSIX 2008 equivalent is `command -v` [2].  We can make the shell
introduction more portable by sticking to the POSIX utilities [3],
especially since `command` is a Bash builtin.

[1]: http://article.gmane.org/gmane.comp.version-control.git/216007
[2]: http://pubs.opengroup.org/onlinepubs/9699919799/utilities/command.html
[3]: http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap04.html#tag_20