Update README.md

Author: ytake

README.md

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 # Nazg\HackHttpExecutor
 
+[![Build Status](https://travis-ci.org/nazg-hack/hack-http-executor.svg?branch=master)](https://travis-ci.org/nazg-hack/hack-http-executor)
+
 This library provides utilities for:
 
  - Emitting [Hack HTTP Request and Response Interfaces](https://github.com/hhvm/hack-http-request-response-interfaces) responses.