Explicitly require bash rather than sh

I ran into a bug on a SmartOS vm. I get this error when trying to invoke `n` because apparently `local` is not defined in `sh` on SmartOS:

    $n
    /home/node/local/nodejs/bin/node
    /home/node/local/bin/n[116]: local: not found [No such file or directory]
    /home/node/local/bin/n[117]: local: not found [No such file or directory]

Here are the versions of each.

    $ sh --version
      version         sh (AT&T Research) 93t+ 2010-03-05

    $ bash --version
    GNU bash, version 4.1.0(1)-release (i386-pc-solaris2.11)
    Copyright (C) 2009 Free Software Foundation, Inc.
    License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
    
    This is free software; you are free to change and redistribute it.
    There is NO WARRANTY, to the extent permitted by law.

I changed the shell script header to require `bash` rather than the more ambiguous `sh`.