Drop invalid -a flag for basename.

basename doesn't accept options in bash. This causes the VERSIONS variable get the 'N/A' value for $ nvm ls command.

From basename man page:
NAME
       basename - strip directory and suffix from filenames

SYNOPSIS
       basename NAME [SUFFIX]
       basename OPTION

DESCRIPTION
       Print NAME with any leading directory components removed.  If specified, also remove a trailing SUFFIX.

       --help display this help and exit

       --version
              output version information and exit



coreutils version used in Linux Mint 14 Nadia.

GNU coreutils 8.12.197-032bb                                               September 2011                                                               BASENAME(1)

No idea if this flag exists on FreeBSD or other unixes (if they use gnu coreutils shouldn't be any problem)
Merge pull request #231 from miguelcoba/patch-1

Drop invalid -a flag for basename.