Adjust phpunit compatiblity for travis

Author: estahn

composer.json

@@ -13,12 +13,12 @@
     ],
     "minimum-stability": "dev",
     "require": {
-        "php": ">=5.4.0",
+        "php": "^5.4 || ^7.0",
         "justinrainbow/json-schema": "^1.6",
         "mtdowling/jmespath.php": "^2.3"
     },
     "require-dev": {
-        "phpunit/phpunit": "^5.2"
+        "phpunit/phpunit": "~4.8|~5.2"
     },
     "autoload": {
         "psr-4": {