[coverage] Replacing coveralls with codecov

[john-bodley]

I'm unsure why but coveralls hasn't run as part of the Travis CI since December 2017. It seems nowadays most repos are using codecov rather than coveralls (including incubator-airflow) which is free and requires no token for public GitHub repos and thus I though there was merit in trying to leverage this.

One really nice thing about codecov is they have a browser extension which works seamlessly with GitHub and allows you to see the code coverage right from within GitHub. See here for more details, which includes a demo on YouTube. What's every sweeter, is you can see code coverage of your PR which helps indicate whether you need to write additional unit tests for your change.

Previously coveralls was integrated with tox which meant passing Travis CI environment variables to tox and making the run_tests.sh script aware of whether it was running in a CI environment. Personally I'm not a fan of this interwoven dependency, i.e., I think of tox as a testing environment which can run locally via shell-based testing or with a CI server, and thus should be agnostic of the CI service. Although codecov can also be integrated with tox, given that codecov is primarily a web-based CI tool, personally it makes sense that this is handled completely within Travis CI.

Note this PR produced the following report.

to: @mistercrunch @xrmx

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@4135854). Click here to learn what that means.
The diff coverage is n/a.

@@           Coverage Diff            @@
##             master   #4533   +/-   ##
========================================
  Coverage          ?   71.1%           
========================================
  Files             ?     188           
  Lines             ?   14803           
  Branches          ?    1083           
========================================
  Hits              ?   10525           
  Misses            ?    4275           
  Partials          ?       3

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[mistercrunch]

This is great. Now we need to figure out how to hook up the JS coverage report.

[mistercrunch]

LGTM