travis: add php 7.3, 7.4, 8.0; remove hhvm

.travis.yml

@@ -5,7 +5,8 @@ dist: trusty
 jobs:
   fast_finish: true
   allow_failures:
-    - php: "hhvm"
+    - php: "7.4snapshot"
+    - php: "nightly"
   include:
     - php: "5.3"
       dist: precise
@@ -15,8 +16,8 @@ jobs:
     - php: "7.0"
     - php: "7.1"
     - php: "7.2"
-    - php: "nightly"
-    - php: "hhvm"
+    - php: "7.3"
+    - php: "7.4snapshot"
 
 env:
   - CLOSURE_VERSION: 20161024