Implement PSR-18

Author: t3ko

composer.json

@@ -12,19 +12,16 @@
   "require": {
     "phpro/soap-client": "0.6.0",
     "sabre/xml": "^2.1",
-    "psr/http-message": "^1.0",
-    "php-http/httplug": "^1.1",
-    "php-http/message-factory": "^1.0",
-    "php-http/discovery": "^1.3",
-    "php-http/client-common": "^1.6",
-    "php-http/client-implementation": "^1.0"
+    "psr/http-client": "^1.0",
+    "psr/http-client-implementation": "*",
+    "php-http/message-factory": "^1.0"
   },
   "require-dev": {
     "php-http/message": "^1.6",
     "zendframework/zend-code": "^3.1.0",
     "php-http/mock-client": "^1.0",
     "guzzlehttp/psr7": "^1.4",
-    "php-http/guzzle6-adapter": "^1.1",
+    "php-http/guzzle6-adapter": "^2",
     "ocramius/package-versions": "^1.1.2",
     "ocramius/proxy-manager": "^2.0.2",
     "php-vcr/php-vcr": "^1.3.2",