Test against PHP 7.4 and PHP 7.3 and update distro on Travis

Author: clue

.travis.yml

@@ -2,21 +2,27 @@ language: php
 
 php:
 # - 5.3 # requires old distro
-  - 5.4
-  - 5.5
+# - 5.4 # requires old distro
+# - 5.5 # requires old distro
   - 5.6
   - 7.0
   - 7.1
   - 7.2
+  - 7.3
+  - 7.4
   - hhvm # ignore errors, see below
 
 # lock distro so new future defaults will not break the build
-dist: trusty
+dist: xenial
 
 matrix:
   include:
     - php: 5.3
       dist: precise
+    - php: 5.4
+      dist: trusty
+    - php: 5.5
+      dist: trusty
   allow_failures:
     - php: hhvm