Add cleanup phase if using specific node version

Have `n` return the version of node to the previous version.

Author: Ilya Radchenko

worker.js

@@ -73,7 +73,6 @@ module.exports = {
 
         async.series(tasks, done);
       }
-      // cleanup: 'rm -rf node_modules'
     };
 
     if (config.test && config.test !== '<none>') {
@@ -97,6 +96,11 @@ module.exports = {
         cmd: 'n ' + (config.fork === 'io.js' ? 'io ' : '') + config.runtime,
         silent: true
       };
+      
+      ret.cleanup = {
+        cmd: 'n prev',
+        silent: true
+      };
     }
 
     cb(null, ret);