Update travis CI config

Author: robocoder

.travis.yml

@@ -9,7 +9,10 @@ php:
   - 5.6
   - 7.0
   - 7.1
-  - hhvm
+  - 7.2
+  - 7.3
+  - 7.4snapshot
+  - nightly
 
 #before_install:
 #  # This update is mandatory or the 'apt-get install' calls following will fail
@@ -30,3 +33,8 @@ script:
 
 after_script:
   - vendor/bin/coveralls
+
+matrix:
+  allow_failures:
+    - php: 7.4snapshot
+    - php: nightly