Add travis badge

Author: Alex Yang

README.md

@@ -1,5 +1,8 @@
 # mock-echo
-[![npm package][npm-badge]][npm]
+[![Travis][build-badge]][build] [![npm package][npm-badge]][npm]
+
+[build-badge]: https://img.shields.io/travis/alexxiyang/mock-echo/master.svg?style=flat-square
+[build]: https://travis-ci.org/alexxiyang/mock-echo
 
 [npm-badge]: https://img.shields.io/npm/v/mock-echo.svg?style=flat-square
 [npm]: https://www.npmjs.org/package/mock-echo