gogs: fix error on push

Pushing to gogs only works if the `gogs` user's shell is `bash`. For error and
solution, refer to [this SO thread](http://stackoverflow.com/a/22315659)

nixos/modules/services/misc/gogs.nix

@@ -208,6 +208,7 @@ in
         group = "gogs";
         home = cfg.stateDir;
         createHome = true;
+        shell = pkgs.bash;
       };
       extraGroups.gogs.gid = config.ids.gids.gogs;
     };