fix: use nanoid instead of shortid

shortid is deprecated and they recommend nanoid instead.
We're not sure if this has to do with possible name
collisions or enumerability, but to be sure and on the
safe side, we're changing this. nanoid seems quite safe
since it uses node's crypto module underneath.

Signed-off-by: Erik Michelson <github@erik.michelson.eu>

lib/models/revision.js

@@ -4,7 +4,7 @@ const Sequelize = require('sequelize')
 const async = require('async')
 const moment = require('moment')
 const childProcess = require('child_process')
-const shortId = require('shortid')
+const nanoid = require('nanoid')
 const path = require('path')
 
 const Op = Sequelize.Op
@@ -44,7 +44,7 @@ function createDmpWorker () {
 
 function sendDmpWorker (data, callback) {
   if (!dmpWorker) dmpWorker = createDmpWorker()
-  const cacheKey = Date.now() + '_' + shortId.generate()
+  const cacheKey = Date.now() + '_' + nanoid.nanoid()
   dmpCallbackCache[cacheKey] = callback
   data = Object.assign(data, {
     cacheKey