replace underscore with our own

we only need like 5 simple functions

src/scheduler.js

@@ -17,7 +17,7 @@ const apps = require('./apps.js'),
     debug = require('debug')('box:scheduler'),
     docker = require('./docker.js'),
     safe = require('safetydance'),
-    _ = require('underscore');
+    _ = require('./underscore.js');
 
 const gState = {}; // appId -> { containerId, schedulerConfig (manifest+crontab), cronjobs }
 const gSuspendedAppIds = new Set(); // suspended because some apptask is running