Update message dependency version

Author: sagikazarmark

composer.json

@@ -14,7 +14,7 @@
         "php": ">=5.4",
         "phpunit/phpunit": "^4.5",
         "php-http/discovery": "^0.5",
-        "php-http/message": "^0.1",
+        "php-http/message": "~0.2.1",
         "php-http/httplug": "~1.0@dev",
         "guzzlehttp/psr7": "^1.0",
         "th3n3rd/cartesian-product": "^0.3"