Pins to Stratigility 3.0.0alpha1

There are changes about to be merged to Stratigility that involve BC
breaks. To ensure consumers of the dev releases of Expressive do not run
into problems, this patch pins to 3.0.0alpha1, against which we know
tests currently pass, and which poses no BC break.

Author: weierophinney

composer.json

@@ -30,7 +30,7 @@
         "zendframework/zend-diactoros": "^1.3.10",
         "zendframework/zend-expressive-router": "^3.0.0-dev",
         "zendframework/zend-expressive-template": "^2.0.0-dev",
-        "zendframework/zend-stratigility": "^3.0.0-dev"
+        "zendframework/zend-stratigility": "^3.0.0alpha1@alpha"
     },
     "require-dev": {
         "filp/whoops": "^1.1.10 || ^2.1.13",