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/Serializer.php

@@ -30,10 +30,10 @@ class Serializer
     protected $isFirstLineIndented = true;
 
     /** @var int|null The max length of a line. */
-    protected $lineLength = null;
+    protected $lineLength;
 
     /** @var DocBlock\Tags\Formatter A custom tag formatter. */
-    protected $tagFormatter = null;
+    protected $tagFormatter;
 
     /**
      * Create a Serializer instance.