graphviz: fix vimdot program

vimdot doesn't work at the moment because one of its dependencies,
'which', is missing; vimdot fails to find gvim or vim and aborts.
Instead of adding a dependency on 'which', replace it with the POSIX
command 'command -v'.
Merge branch 'graphviz-vimdot-fix' of git://github.com/bjornfor/nixpkgs

Branch contains a trivial fix.

Signed-off-by: Shea Levy <shea@shealevy.com>