feat: switch to newer changes stream package

this new version is maintained by npm and comes with many improvements,
one of which is a significant performance improvement that uses linear
instead of quadratic memory when catching up from early update sequences.

Author: janl

lib/follow.js

@@ -2,7 +2,7 @@ const crypto = require('crypto')
 const url = require('url')
 
 const _ = require('lodash')
-const ChangesStream = require('changes-stream')
+const ChangesStream = require('@npmcorp/changes-stream')
 const request = require('request')
 const StatsD = require('hot-shots')
 const cleanDoc = require('normalize-registry-metadata')