Require stable php-pm (#77)

* Require stable php-pm

* Add license

Author: andig

composer.json

@@ -1,9 +1,8 @@
 {
     "name": "php-pm/httpkernel-adapter",
-    "minimum-stability": "dev",
-    "prefer-stable": true,
+    "license": "MIT",
     "require": {
-        "php-pm/php-pm": "*",
+        "php-pm/php-pm": "^1.0",
         "symfony/http-foundation": "^2.6|^3.0",
         "symfony/http-kernel": "^2.6|^3.0",
         "ringcentral/psr7": "^1.2"