perf(Promise): use Bluebird as default implementation

bin/xo-server

@@ -7,6 +7,10 @@
 // Better stack traces if possible.
 require('../better-stacks')
 
+// Use Bluebird for all promises as it provides better performance and
+// less memory usage.
+global.Promise = require('bluebird')
+
 // Make unhandled rejected promises visible.
 process.on('unhandledRejection', function (reason) {
   console.warn('[Warn] Possibly unhandled rejection:', reason && reason.stack || reason)