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

src/DocBlock/DescriptionFactory.php

@@ -108,7 +108,7 @@ class DescriptionFactory
      *
      * @return string[]|Tag[]
      */
-    private function parse($tokens, TypeContext $context)
+    private function parse($tokens, ?TypeContext $context = null): array
     {
         $count = count($tokens);
         $tagCount = 0;