gitlab: add meta data

pkgs/applications/version-management/gitlab/default.nix

···
     inherit rubyEnv;
     ruby = rubyEnv.wrappedRuby;
   };
+
+  meta = with stdenv.lib; {
+    description = "Web-based Git-repository manager";
+    homepage = https://gitlab.com;
+    license = licenses.mit;
+  };
 }