Move PHPStan configuration to its file

.travis.yml

@@ -35,7 +35,7 @@ before_script:
 
 script:
   - composer validate
-  - if [[ $STATIC_ANALYSIS = true ]]; then ./vendor/bin/phpstan analyse src --level=2; fi
+  - if [[ $STATIC_ANALYSIS = true ]]; then ./vendor/bin/phpstan analyse; fi
   - |
     if [[ $VALIDATION = true ]]; then
       ./vendor/bin/phpunit --testsuite validation

phpstan.neon

@@ -1,4 +1,7 @@
 parameters:
+    level: 2
+    paths:
+        - src/
     ignoreErrors:
         # phpstan issue, see: https://github.com/phpstan/phpstan/issues/1306
         - "/Variable .unaryExpression might not be defined./"