Resolves #158. Manually require psr/http-factory

Author: erickskrauch

composer.json

@@ -23,7 +23,8 @@
         "ext-simplexml": "*",
         "psr/cache": "^1.0",
         "psr/http-client": "^1.0",
-        "psr/http-message": "^1.0"
+        "psr/http-message": "^1.0",
+        "psr/http-factory": "^1.0"
     },
     "require-dev": {
         "phpunit/phpunit": "^5.0.5",