Add PHPStan and bump to max level (#139)

* composer: put require- together

* add phpstan

* add phpstan to travis

* phpstan fixes

* phpstan fixes

* phpstan fixes

* drop null defaults since every property is null by default

* DescriptionFactory: fix parse nullable type

* travis: add phpstan max

* travis: fix phpstan config

composer.json

@@ -15,6 +15,12 @@
         "phpdocumentor/type-resolver": "^0.4",
         "webmozart/assert": "^1.0"
     },
+    "require-dev": {
+        "mockery/mockery": "^1.0",
+        "phpunit/phpunit": "^6.4",
+        "doctrine/instantiator": "^1.0",
+        "phpstan/phpstan": "^0.9.0"
+    },
     "autoload": {
         "psr-4": {
             "phpDocumentor\\Reflection\\": "src"
@@ -25,11 +31,6 @@
             "phpDocumentor\\Reflection\\": "tests/unit"
         }
     },
-    "require-dev": {
-        "mockery/mockery": "^1.0",
-        "phpunit/phpunit": "^6.4",
-        "doctrine/instantiator": "^1.0"
-    },
     "extra": {
         "branch-alias": {
             "dev-master": "4.x-dev"