Bump Event Machine version

Author: codeliner

composer.json

@@ -18,7 +18,7 @@
   "require": {
     "php": "^7.1",
     "roave/security-advisories": "dev-master",
-    "proophsoftware/event-machine": "^v0.20",
+    "proophsoftware/event-machine": "^v0.21",
     "proophsoftware/postgres-document-store": "^0.3",
     "prooph/pdo-event-store": "^1.0",
     "prooph/humus-amqp-producer": "^2.0",