Check syntax of PHP files in src

.travis.yml

@@ -71,6 +71,7 @@ before_script:
   - git config --global user.email travis@example.com
 
 script:
+  - ( set -e; for f in $(find src/ -name *.php); do php -l "$f" || exit 1; done )
   - if [[ $PHPSTAN == "1" ]]; then
       bin/composer require --dev phpstan/phpstan:^0.12 phpunit/phpunit:^7.5 --no-update &&
       bin/composer update phpstan/* phpunit/* sebastian/* --with-all-dependencies &&