Add support for sharedb@4

Preemptive update for upcoming sharedb release

Author: craigbeck

package.json

@@ -5,7 +5,7 @@
   "main": "index.js",
   "dependencies": {
     "redis": "^2.6.0 || ^3.0.0",
-    "sharedb": "^1.0.0 || ^2.0.0 || ^3.0.0"
+    "sharedb": "^1.0.0 || ^2.0.0 || ^3.0.0 || ^4.0.0"
   },
   "devDependencies": {
     "chai": "^4.2.0",