use codecov instead of coveralls

.travis.yml

@@ -4,4 +4,6 @@ node_js:
   - "5"
 sudo: false
 script: "make test-travis"
-after_script: "npm install coveralls@2 && cat ./coverage/lcov.info | coveralls"
+after_script:
+  - npm install codecov
+  - ./node_modules/.bin/codecov

Readme.md

@@ -162,8 +162,8 @@ See [AUTHORS](AUTHORS).
 [npm-url]: https://npmjs.org/package/koa
 [travis-image]: https://img.shields.io/travis/koajs/koa/master.svg?style=flat-square
 [travis-url]: https://travis-ci.org/koajs/koa
-[coveralls-image]: https://img.shields.io/coveralls/koajs/koa/master.svg?style=flat-square
-[coveralls-url]: https://coveralls.io/r/koajs/koa?branch=master
+[coveralls-image]: https://codecov.io/github/koajs/koa/coverage.svg?branch=master
+[coveralls-url]: https://codecov.io/github/koajs/koa?branch=master
 [gitter-image]: https://badges.gitter.im/Join%20Chat.svg
 [gitter-url]: https://gitter.im/koajs/koa?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge
 [#koajs]: https://webchat.freenode.net/?channels=#koajs