Enhancement: Require at least phpstan/phpstan:~0.11.7

[localheinz]

This PR

• requires at least phpstan/phpstan:~0.11.7
• fixes issues reported via static analysis

[codecov]

Codecov Report

Merging #91 into master will decrease coverage by 16.1%.
The diff coverage is 20%.

@@              Coverage Diff              @@
##             master      #91       +/-   ##
=============================================
- Coverage       100%   83.89%   -16.11%     
- Complexity       70       82       +12     
=============================================
  Files            13       13               
  Lines           238      298       +60     
=============================================
+ Hits            238      250       +12     
- Misses            0       48       +48

Impacted Files	Coverage Δ	Complexity Δ
...rc/Methods/NoNullableReturnTypeDeclarationRule.php	80% <20%> (-20%)	5 <0> (+1)
...rc/Methods/NoParameterWithNullDefaultValueRule.php	88.57% <20%> (-11.43%)	7 <0> (+1)
...res/NoParameterWithNullableTypeDeclarationRule.php	80% <20%> (-20%)	5 <0> (+1)
src/Classes/NoExtendsRule.php	84.61% <20%> (-15.39%)	7 <0> (+1)
...ods/NoConstructorParameterWithDefaultValueRule.php	87.5% <20%> (-12.5%)	7 <0> (+1)
...ods/NoParameterWithNullableTypeDeclarationRule.php	87.87% <20%> (-12.13%)	6 <0> (+1)
...c/Closures/NoParameterWithNullDefaultValueRule.php	81.81% <20%> (-18.19%)	6 <0> (+1)
.../Functions/NoParameterWithNullDefaultValueRule.php	83.33% <20%> (-16.67%)	6 <0> (+1)
.../Functions/NoNullableReturnTypeDeclarationRule.php	69.23% <20%> (-30.77%)	5 <0> (+1)
...ons/NoParameterWithNullableTypeDeclarationRule.php	81.81% <20%> (-18.19%)	5 <0> (+1)
... and 2 more

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