package.json: remove codecov from devDependencies

There is no need to install codecov on developer machines.
To enable it in Travis, we only need to install it in the Travis
setup. This would help us remove some unnecessary entries in
package-lock.json as well.

Author: joyeecheung

.travis.yml

@@ -10,4 +10,5 @@ script:
   - npm run lint
   - npm run coverage-all
 after_success:
+  - npm install codecov
   - npm run report-coverage