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"