Use --ignore-platform-reqs on Travis

Allows testing on nightly, which is PHP 8.

Author: nikic

.travis.yml

@@ -16,7 +16,7 @@ php:
 
 install:
   - if [ $TRAVIS_PHP_VERSION = '7.0' ]; then composer require satooshi/php-coveralls '~1.0'; fi
-  - composer install --prefer-dist
+  - composer install --prefer-dist --ignore-platform-reqs
 
 matrix:
   allow_failures: