Test against PHP 7.3 and use legacy PHPUnit 5 on legacy HHVM

.travis.yml

@@ -8,7 +8,8 @@ php:
   - 7.0
   - 7.1
   - 7.2
-  - hhvm # ignore errors, see below
+  - 7.3
+# - hhvm # requires legacy phpunit & ignore errors, see below
 
 # lock distro so new future defaults will not break the build
 dist: trusty
@@ -17,6 +18,8 @@ matrix:
   include:
     - php: 5.3
       dist: precise
+    - php: hhvm
+      install: composer require phpunit/phpunit:^5 --dev --no-interaction
   allow_failures:
     - php: hhvm