Use the latest version of discovery. (#842)

Author: Nyholm

composer.json

@@ -19,7 +19,7 @@
         "php-http/message-factory": "^1.0.2",
         "psr/http-message-implementation": "^1.0",
         "php-http/client-implementation": "^1.0",
-        "php-http/discovery": "^1.0"
+        "php-http/discovery": "^1.4"
     },
     "require-dev": {
         "phpunit/phpunit": "6.3.*",