Drop hhvm, has been failing for a long time, doesn't seem interest to fix it. wastes resources.

.travis.yml

@@ -6,10 +6,6 @@ php:
 - 7.1
 - 7.2
 - 7.3
-- hhvm
-matrix:
-  allow_failures:
-  - php: hhvm
 before_script:
 - composer update
 - if [[ "$TRAVIS_PHP_VERSION" == "7.0" ]]; then composer require --dev vimeo/psalm:0.3.92; fi