Bumped minimum client common version

Author: GrahamCampbell

composer.json

@@ -23,7 +23,7 @@
 	"require": {
 		"php": "^5.6 || ^7.0",
 		"ext-xml": "*",
-		"php-http/client-common": "^1.5",
+		"php-http/client-common": "^1.6",
 		"php-http/client-implementation": "^1.0",
 		"php-http/discovery": "^1.2",
 		"php-http/httplug": "^1.1",