Drop support for PHP under 7.0 in composer files

Author: WyriHaximus

composer.json

@@ -12,13 +12,13 @@
         }
     ],
     "require": {
-        "php": "^7.0 || ^5.5",
+        "php": "^7.0",
         "psr/http-message": "^1.0"
     },
     "require-dev": {
         "guzzlehttp/psr7": "^1.3",
         "humbug/humbug": "1.0.0-alpha2",
-        "phpunit/phpunit": "^5.5 || ^4.8",
+        "phpunit/phpunit": "^5.6",
         "squizlabs/php_codesniffer": "^2.6"
     },
     "autoload": {
@@ -34,7 +34,7 @@
     "config": {
         "sort-packages": true,
         "platform": {
-            "php": "5.5.9"
+            "php": "7.0"
         }
     },
     "scripts": {