Update phpstan

.travis.yml

@@ -93,7 +93,7 @@ script:
 
   - if [[ $CODECOVERAGE = 1 ]]; then phpdbg -qrr vendor/bin/phpunit --coverage-clover=clover.xml; fi
 
-  - if [[ $CHECKS = 1 ]]; then composer require --dev "phpstan/phpstan:^0.10" && vendor/bin/phpstan analyse -c phpstan.neon -l 2 src/; fi
+  - if [[ $CHECKS = 1 ]]; then composer require --dev phpstan/phpstan:^0.11 && vendor/bin/phpstan analyse -c phpstan.neon -l 2 src/; fi
   - if [[ $CHECKS = 1 ]]; then composer cs-check; fi
 
 after_success: