Remove non-existent dependency

Author: welcoMattic

composer.json

@@ -23,8 +23,7 @@
         "php-http/client-common": "^1.2",
         "php-http/cache-plugin": "^1.0",
         "php-http/logger-plugin": "^1.0",
-        "php-http/discovery": "^1.0",
-        "php-http/client-implementation": "^1.0"
+        "php-http/discovery": "^1.0"
     },
     "require-dev": {
         "phpunit/phpunit": "^4.5 || ^5.4",