On Debian 7 (and probably relative distro's) `aptitude show virtualenv` exits
with 0, since it is a virtual package. However, it doesn't have any installation
candidates, so filter on this case before trying to install `virtualenv` to
prevent installation-errors while bootstrapping.
NB, to make this clear:
(0)#: apt-cache show virtualenv
N: Can't select versions from package 'virtualenv' as it is purely virtual
N: No packages found
(0)#: echo $?
0
Furthermore, --quiet=0 is necessary, to be able to grep through `apt-cache`'s
output via a pipe. More details on
http://unix.stackexchange.com/questions/201869/why-isnt-apt-cache-policy-output-piped/202041#202041.
