ISpawnOptions.cmd is optional

These options are perfectly valid but will cause a ts build to fail: grunt.util.spawn({grunt: true, args: ['"mytask"]});

gruntjs/gruntjs.d.ts

@@ -1172,7 +1172,7 @@ declare module grunt {
             /**
              * The command to execute. It should be in the system path.
              */
-            cmd: string
+            cmd?: string
 
             /**
              * If specified, the same grunt bin that is currently running will be