Try to fix phpstan errors

.travis.yml

@@ -33,6 +33,7 @@ matrix:
       env:
         - deps=high
         - PHPSTAN=1
+    - php: nightly
   fast_finish: true
   allow_failures:
     - php: nightly
@@ -64,7 +65,7 @@ script:
   - ./vendor/bin/simple-phpunit
   # Run PHPStan
   - if [[ $PHPSTAN == "1" ]]; then
-      bin/composer require --dev phpstan/phpstan:^0.12 &&
+      bin/composer require --dev phpstan/phpstan:^0.12 phpunit/phpunit:^6.5 &&
       vendor/bin/phpstan analyse --configuration=phpstan/config.neon;
     fi