Travis: add php 7.3, php 7.4, drop hhvm

Author: glensc

.travis.yml

@@ -10,13 +10,13 @@ cache:
 jobs:
   fast_finish: true
   allow_failures:
-    - php: "hhvm"
     - php: "nightly"
   include:
     - php: "7.1"
     - php: "7.2"
+    - php: "7.3"
+    - php: "7.4"
     - php: "nightly"
-    - php: "hhvm"
 
 install:
   - composer install