Chore: change coverage service (coverall to codecov)

  The reason of change mentioned on chaijs#927
  This patch:
    - add codecov as devDep, as it's not pre-installed by travis.
    - add codecov script on travis.
    - change badge link (coverall to codecov), and point the image to chai account on codecov.
  Fix chaijs#927

.travis.yml

@@ -10,7 +10,8 @@ node_js:
  - node # safety net; don't remove
 
 script:
-- make test-travisci
+  - make test-travisci
+  - codecov
 
 env:
   global:

README.md

@@ -49,10 +49,10 @@
       src="https://img.shields.io/travis/chaijs/chai/master.svg?style=flat-square"
     />
   </a>
-  <a href="https://coveralls.io/r/chaijs/chai">
+  <a href="https://codecov.io/gh/chaijs/chai">
     <img
       alt="coverage:?"
-      src="https://img.shields.io/coveralls/chaijs/chai/master.svg?style=flat-square"
+      src="https://img.shields.io/codecov/c/github/chaijs/chai.svg?style=flat-square"
     />
   </a>
   <a href="">

package.json

@@ -43,6 +43,7 @@
   "devDependencies": {
     "browserify": "^14.4.0",
     "bump-cli": "^1.1.3",
+    "codecov": "^3.0.0",
     "istanbul": "^0.4.3",
     "karma": "^1.0.0",
     "karma-firefox-launcher": "^1.0.0",