update composer for beta release

Author: prolic

composer.json

@@ -28,7 +28,7 @@
     "require": {
         "php": "^7.1",
         "container-interop/container-interop": "^1.1",
-        "prooph/event-store" : "7.0.x-dev",
+        "prooph/event-store" : "^7.0.0-beta",
         "roave/security-advisories": "dev-master",
         "zendframework/zend-diactoros": "^1.3.7",
         "zendframework/zend-expressive": "^1.0",