Add codecov reporting to build process.

[dopry]

- Summary
depends on: #610
closes: #609

This PR will add codecov reporting to the build process.

For this to properly work someone will need to

1. goto https://codecov.io/gh/netlify/netlify-cms/settings and authorize codecov
2. copy the repository upload token.
3. goto https://travis-ci.org/netlify/netlify-cms/settings
4. add an environment variable named CODECOV_TOKEN with the value of the repository upload token.

- Test plan

Do the above and then merge. The following master build should update the baseline info in codecov. In my experience it takes a few PRs and builds for codecov to start reporting properly.

- Description for the changelog

Add codecov reporting to the build process.

[medfreeman]

I think you only need to perform step 1, as uploading to codecov auth is automatically handled for travis without needing to provide a token (it's said on codecov but not so clear).

[erquhart]

LGTM, but withholding approval until #610 is merged.

[erquhart]

Authorized codecov and restarted the CI build.

[Benaiah]

LGTM. It's useful, though I'd like to give it a bit of time before we make an hard requirements on PRs based on its output.

[erquhart]

@Benaiah agreed.