remove bithound badge (does not work with ESLint 2.0)

Author: yyx990803

README.md

@@ -3,7 +3,6 @@
 <p align="center">
   <a href="https://circleci.com/gh/vuejs/vue/tree/master"><img src="https://circleci.com/gh/vuejs/vue/tree/master.svg?style=shield" alt="Build Status"></a>
   <a href="https://codecov.io/github/vuejs/vue?branch=master"><img src="https://codecov.io/github/vuejs/vue/coverage.svg?branch=dev" alt="Coverage Status"></a>
-  <a href="https://www.bithound.io/github/vuejs/vue"><img src="https://www.bithound.io/github/vuejs/vue/badges/code.svg" alt="bitHound Code"></a>
   <br>
   <a href="http://issuestats.com/github/vuejs/vue"><img src="http://issuestats.com/github/vuejs/vue/badge/issue?style=flat"></a>
   <a href="https://gitter.im/vuejs/vue"><img src="https://badges.gitter.im/Join Chat.svg"></a>