Prefer stable packages (#376)

* Prefer stable

* fix phpstan

Author: rvanlaak

composer.json

@@ -57,6 +57,7 @@
         "test-ci": "vendor/bin/phpunit --coverage-text --coverage-clover=build/coverage.xml Tests/Unit"
     },
     "minimum-stability": "dev",
+    "prefer-stable": true,
     "extra": {
         "branch-alias": {
             "dev-master": "0.10-dev"