Fixes issue 232 and "nvm ls" returning "N/A" when it shouldn't

Fixes issue 232 that occurs in at least bash v4.2.25 where "nvm ls" returns "N/A" no matter how many versions of node have been installed.  The fix uses a combination of the find, tr and basename commands instead of basename only.  Tested in bash v3.2.48 and v4.2.25 and zsh v4.3.11.