Explanation

It has been a few days after my #5244 was merged and yet, Travis was still installing codecov and pyyaml on start. Pics from build 11932.1 on develop:

This means that codecov and pyyaml was not cached at all.

In the spirit of careful investigation for every line of code written, I dig up the related #1640 and its parent issue #1636. Toward the end of #1636 thread, pip cache was dropped due to instability and ruled ineffective due to variable time.

We have since grown from running 3 jobs to 13 jobs. Every seconds saved is tangible imo. I started to look into why pip modules are not getting cached. Here is my finding. I then made the changes and tested on my own Oppia's Travis. Here is pic from my Travis's job 138.1 of branch viet_branch_5222:

As you can see, cached pyyaml shaved 8 secs off that one job. 8 x 13 = 104 secs, less than 2 minutes for each PR. In the grand scheme of things, 2 minutes are not a lot but is still worthy of a PR for me!

Checklist

• The PR title starts with "Fix #bugnum: ", followed by a short, clear summary of the changes.
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