chore: Update supported PHP/OS versions in Travis CI

Testing with PHP 7.2 - 8.1 and hhvm

- PHP 7.0 and 7.1 can be removed, per composer.json PHP require
- HHVM does not work - could be restored if still supported?

.travis.yml

@@ -1,12 +1,12 @@
-dist: trusty
+dist: focal
 language: php
 
 php:
-  - 7.0
-  - 7.1
   - 7.2
   - 7.3
-  - hhvm
+  - 7.4
+  - 8.0
+  - 8.1
 
 # This triggers builds to run on the new TravisCI infrastructure.
 # See: http://docs.travis-ci.com/user/workers/container-based-infrastructure/
@@ -17,11 +17,6 @@ cache:
   directories:
     - $HOME/.composer/cache
 
-matrix:
-  include:
-    - php: 5.6
-      env: 'COMPOSER_FLAGS="--prefer-stable --prefer-lowest"'
-
 before_script:
   - travis_retry composer update ${COMPOSER_FLAGS} --no-interaction --prefer-dist