Merge pull request #197 from GrahamCampbell/patch-2

Added explicit psr/http-client dependency

Author: dbu

composer.json

@@ -15,6 +15,7 @@
         "php-http/httplug": "^2.0",
         "php-http/message-factory": "^1.0",
         "php-http/message": "^1.6",
+        "psr/http-client": "^1.0",
         "psr/http-factory": "^1.0",
         "psr/http-message": "^1.0",
         "symfony/options-resolver": "^2.6 || ^3.4.20 || ~4.0.15 || ~4.1.9 || ^4.2.1 || ^5.0",