Updated composer.json

Required PHP version is changed from 5.4 to 5.5.

Author: Justinas Pošiūnas

composer.json

@@ -6,7 +6,7 @@
     "keywords": ["mailerlite", "sdk", "email", "marketing"],
     "type": "library",
     "require": {
-        "php" : "^5.4|^7.0",
+        "php" : "^5.5|^7.0",
         "guzzlehttp/psr7": "~1.2",
         "php-http/httplug": "^1.0",
         "php-http/curl-client": "^1.4",