Test against legacy PHP 5.3 through PHP 7.3 and support PHPUnit 7

Author: clue

.travis.yml

@@ -5,7 +5,10 @@ php:
   - 5.4
   - 5.5
   - 5.6
-  - 7
+  - 7.0
+  - 7.1
+  - 7.2
+  - 7.3
   - hhvm # ignore errors, see below
 
 # lock distro so future defaults will not break the build

composer.json

@@ -26,7 +26,7 @@
     "require-dev": {
         "clue/arguments": "^2.0",
         "clue/commander": "^1.2",
-        "phpunit/phpunit": "^6.4 || ^5.7 || ^4.8.35"
+        "phpunit/phpunit": "^7.0 || ^6.4 || ^5.7 || ^4.8.35"
     },
     "config": {
         "sort-packages": true