Remove HHVM, not all required extensions are available. Also build against PHP 7.3

Author: willemstuursma

.travis.yml

@@ -11,7 +11,7 @@ php:
   - 7.0
   - 7.1
   - 7.2
-  - hhvm
+  - nightly
 
 env: 
   global:
@@ -29,7 +29,7 @@ services:
 matrix:
   fast_finish: true
   allow_failures:
-    - php: hhvm
+    - php: nightly
 
 before_install:
   - phpenv config-add tests/php-travis.ini