Revert "PHP 7.3 can no longer fail"

This reverts commit cf00f3f.

Author: tvdijen

.travis.yml

@@ -12,6 +12,10 @@ php:
 env:
   - SIMPLESAMLPHP_VERSION=1.17.*
 
+matrix:
+  allow_failures:
+    - php: 7.3
+
 before_script:
   - composer require "simplesamlphp/simplesamlphp:${SIMPLESAMLPHP_VERSION}" --no-update
   - composer update --no-interaction