[Bug fix] Reaching maximum calls stack when copying large number of non-js files in babel-cli (#7320)

Maximum call stack occurs when you try to copy large number of non-js files using `babel-cli@7.0.0-beta.38` or `babel-cli@7.0.0-beta.39`

packages/babel-cli/src/babel/dir.js

@@ -13,7 +13,7 @@ export default function(commander, filenames, opts) {
       base = undefined;
     }
     if (!util.isCompilableExtension(relative, commander.extensions)) {
-      return callback();
+      return process.nextTick(callback);
     }
 
     // remove extension and then append back on .js