Bump up phpstan level.

.travis.yml

@@ -93,7 +93,7 @@ script:
 
   - if [[ $PHPCS = 1 ]]; then composer cs-check; fi
   - if [[ $PHPCS = 3 ]]; then vendor/bin/phpcs -p -s --extensions=php ./src ./tests; fi
-  - if [[ $PHPSTAN = 1 ]]; then composer require --dev phpstan/phpstan:^0.6 && vendor/bin/phpstan analyse -c phpstan.neon -l 0 src; fi
+  - if [[ $PHPSTAN = 1 ]]; then composer require --dev phpstan/phpstan:^0.6 && vendor/bin/phpstan analyse -c phpstan.neon -l 1 src; fi
 
 after_success:
   - if [[ $DEFAULT = 1 && $TRAVIS_PHP_VERSION = 7.0 ]]; then bash <(curl -s https://codecov.io/bash); fi