🏗 Use xenial VMs on Travis for faster bootup

[rsimha]

This PR switches the Travis VMs we use from Trusty to Xenial.

Advantages:

• More recent Linux version (16.04 vs 14.04)
• Fewer unnecessary services are installed by default
• Faster VM bootup time (Xenial: ~20s, Trusty: ~40s)

References:

• https://docs.travis-ci.com/user/reference/overview/#virtualization-environments
• https://docs.travis-ci.com/user/reference/xenial/#services-disabled-by-default

Changes in this PR:

• Change dist in .travis.yml from trusty to xenial
• Consolidate pip install of Python modules (in --user mode, to work without sudo)
• Override Xenial's default Java 11 version (AMP needs Java 8) (OracleJDK 8 install issue on Xenial travis-ci/travis-ci#10290)
• Consolidate addons section
• Remove bundler cache (it was used for Ruby gems, and is unnecessary)
• Replace pip caching (which doesn't work if language is not python), with directory-level caching of $HOME/.cache/pip
• Remove the yarn upgrade step since Xenial uses the latest version of yarn (1.13.0 as of today)

Related to #20887

[rsimha]

I ran some tests, and the Xenial VM boot up time (~20 seconds) is half that of Trusty (~40 seconds).

[rsimha]

/to @estherkim @danielrozenberg This is now ready for review.

[danielrozenberg]

A quick glance at https://travis-ci.org/ampproject/amphtml/pull_requests makes it look like this results in slower builds (~10 minutes in total) - I haven't done a complete evaluation though... not sure how to measure this over a longer period

[rsimha]

When I looked at the logs, the change in total running time appeared to arise from a high variance in the running time for git clone. It's being tracked by #21175.

Meanwhile, I was able to measure a consistent reduction in boot time (not part of total job running time) from ~40s to ~20s. In addition, it's preferable to use a newer version of Linux instead of one that's several years old.

Good point though. We should keep track of total job running time and try to address the steps that are particularly slow. I believe the Sauce Labs job is particularly notorious for being slow when tests start timing out.

[estherkim]

Pretty cool!! I added non-blocking comments.

One thing that could be blocking, though, is if the bundle size bot can't talk to the new VM (the check is pending right now). Not sure how to switch to Xenial and update BSB at the same time.

Also can you related this PR to this issue? #20887

[rsimha]

Bundle-size bot is okay. I believe this problem stemmed from the fact that I un-pushed the last commit to this PR. I've re-run the job and will merge after it goes green.