add phpstan

Author: ashnazg

composer.json

@@ -14,7 +14,8 @@
     },
     "require-dev": {
         "phpunit/phpunit": "^6.5",
-        "mockery/mockery": "^1.0"
+        "mockery/mockery": "^1.0",
+        "phpstan/phpstan": "^0.9.0"
     },
     "autoload": {
         "psr-4": {