When the GNU version of `ls` is not installed (e.g. on a new Mac), the
previous alias causes problems because the --group-directories-first
option is not supported and makes `ls` fail. This commit fixes that by
checking which version is installed and creating the alias accordingly.
Since now the .bashrc is source from .bash_profile, the end result in
terms of what gets executed is the same. However, it is quicker to type
`vim .bashrc` when changing settings.
