Get Travis going for PHP 7.1-7.3

.travis.yml

@@ -1,22 +1,19 @@
 language: php
 
 php:
-  - 5.3
-  - 5.4
-  - 5.5
-  - 5.6
   - 7.0
+  - 7.1
+  - 7.2
+  - 7.3
   - nightly
-  - hhvm
 
 matrix:
   allow_failures:
     - php: nightly
-    - php: hhvm
 
 before_script:
   - composer self-update
   - composer update
 
 script: 
-  - phpunit
+  - vendor/bin/phpunit

composer.json

@@ -54,6 +54,7 @@
             "zetacomponents/base": "~1.8"
         },
         "require-dev": {
+            "phpunit/phpunit": "~7.5",
             "zetacomponents/unit-test": "*"
         }
 }