w3m: fix package name

nix-repl> builtins.parseDrvName "w3m-v0.5.3+git20161120"
{ name = "w3m-v0.5.3+git20161120"; version = ""; }

nix-repl> builtins.parseDrvName "w3m-0.5.3+git20161120"
{ name = "w3m"; version = "0.5.3+git20161120"; }