use full basename (permit periods) in pyvenv-virtual-env-name

Jorgen-

Thanks for pyvenv. 

I name virtualenvs like default-2.7, default-3.3, default-3.4, etc. The use of file-name-base in pyvenv.el caused pyvenv-virtual-env-name to be truncated at the first period. This patch uses file-name-nondirectory to preserve the period (i.e., more like basename in GNU coreutils).

Cheers,
Reece