Integrate codecov.

Author: trekhleb

.travis.yml

@@ -4,7 +4,7 @@ language: node_js
 node_js:
   - node
 install:
-  - npm install -g codecov
+  - npm i -g eslint codecov
 script:
   - npm run ci
   - codecov

README.md

@@ -1,6 +1,7 @@
 # JavaScript Algorithms and Data Structures
 
 [![Build Status](https://travis-ci.org/trekhleb/javascript-algorithms.svg?branch=master)](https://travis-ci.org/trekhleb/javascript-algorithms)
+[![codecov](https://codecov.io/gh/trekhleb/javascript-algorithms/branch/master/graph/badge.svg)](https://codecov.io/gh/trekhleb/javascript-algorithms)
 
 ## [Data Structures](https://github.com/trekhleb/javascript-algorithms/tree/master/src/data-structures)