Bump discovery minimum version

Author: barryvdh

composer.json

@@ -40,7 +40,7 @@
     "require": {
         "php": "^7.1",
         "moneyphp/money": "^3.1",
-        "php-http/discovery": "^1.3",
+        "php-http/discovery": "^1.6",
         "psr/http-client": "^1",
         "psr/http-client-implementation": "^1",
         "psr/http-factory": "^1",