This reverts commit b800654a.

This time, do so in Travis' sudo-enabled environment,
which is said to be less prone to networking issues.
It also means we'll run in an env with 7.5GB of RAM
instead of 4GB or RAM (at the cost of about 45 seconds
of extra boot-up time).

https://github.com/flutter/flutter/issues/10940

This reverts commit aa904a58.

Fixes #10940