Fix tests

http://hydra.nixos.org/build/4902041

src/lib/Hydra/Helper/Nix.pm

@@ -4,6 +4,7 @@ use strict;
 use Exporter;
 use File::Path;
 use File::Basename;
+use Config::General;
 use Hydra::Helper::CatalystUtils;
 use Hydra::Model::DB;