Added php-http/httplug 2.0 support

Author: vinkla

composer.json

@@ -19,14 +19,14 @@
         "php": "^7.2",
         "php-http/client-implementation": "^1.0",
         "php-http/discovery": "^1.4",
-        "php-http/httplug": "^1.1",
+        "php-http/httplug": "^1.1 || ^2.0",
         "php-http/message-factory": "^1.0"
     },
     "require-dev": {
         "php-http/guzzle6-adapter": "^1.1",
-        "php-http/message": "^1.6",
+        "php-http/message": "^1.7",
         "php-http/mock-client": "^1.1",
-        "phpunit/phpunit": "^7.2"
+        "phpunit/phpunit": "^7.4"
     },
     "extra": {
         "branch-alias": {