chore(infrastructure): Fix master diff failures on Travis CI

[acdvorak]

What it does

• Fixes Screenshot test CI task hanging/killed while comparing against master #3555:
  • Shards master image diffing into 50 parallel comparisons at a time instead of comparing all 541 images at once
• Reduces startup time by ~1 minute:
  • Prefetches image files in parallel instead of sequentially
  • Removes unnecessary URL prefetching (e.g., HTML files)
• Tries to prevent Travis CI job logs from getting truncated too early (e.g., job #766.5):
  • Adds sleep 5 to after_script in .travis.yml (see this GitHub issue)

How to test

1. Create a PR that modifies golden.json
2. The Travis CI job should pass, and its log output should have something like this near the bottom:

   Comparing 541 screenshots to master
   
   [+ 12,500ms] Comparing screenshots 1–50...
   [+  5,997ms] Comparing screenshots 51–100...
   

Example output

Before:

https://travis-ci.com/material-components/material-components-web/jobs/145553262

After:

https://travis-ci.com/material-components/material-components-web/jobs/147092934

[codecov-io]

Codecov Report

Merging #3598 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3598   +/-   ##
=======================================
  Coverage   98.43%   98.43%           
=======================================
  Files         120      120           
  Lines        5182     5182           
  Branches      645      645           
=======================================
  Hits         5101     5101           
  Misses         81       81

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7b6d86b...3cc082a. Read the comment docs.

[mdc-web-bot]

🤖 Beep boop!

Screenshot test report 🚦

4 screenshots changed from master on commit 723aeb4:

• https://storage.googleapis.com/mdc-web-screenshot-tests/travis/2018/09/20/21_37_58_993/report/report.html?utm_source=github&utm_content=pr_comment

Details

4 Changed:

• spec/mdc-fab/classes/mini.html

[mdc-web-bot]

🤖 Beep boop!

Screenshot test report 🚦

4 screenshots changed from master on commit 6bc09ca:

• https://storage.googleapis.com/mdc-web-screenshot-tests/travis/2018/09/20/22_48_45_085/report/report.html?utm_source=github&utm_content=pr_comment

Details

4 Changed:

• spec/mdc-fab/classes/mini.html

[mdc-web-bot]

🤖 Beep boop!

Screenshot test report 🚦

4 screenshots changed from master on commit d45889c:

• https://storage.googleapis.com/mdc-web-screenshot-tests/travis/2018/09/20/23_13_34_690/report/report.html?utm_source=github&utm_content=pr_comment

Details

4 Changed:

• spec/mdc-fab/classes/mini.html

[mdc-web-bot]

🤖 Beep boop!

Screenshot test report 🚦

4 screenshots changed from master on commit 1326da4:

• https://storage.googleapis.com/mdc-web-screenshot-tests/travis/2018/09/20/23_34_05_248/report/report.html?utm_source=github&utm_content=pr_comment

Details

4 Changed:

• spec/mdc-fab/classes/mini.html

[mdc-web-bot]

All 541 screenshot tests passed for commit ddc6050 vs. master! 💯🎉

[mdc-web-bot]

All 541 screenshot tests passed for commit 9a42b9e vs. master! 💯🎉

[codecov-io]

Codecov Report

Merging #3598 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3598   +/-   ##
=======================================
  Coverage   98.43%   98.43%           
=======================================
  Files         120      120           
  Lines        5182     5182           
  Branches      645      645           
=======================================
  Hits         5101     5101           
  Misses         81       81

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7b6d86b...d66ae4a. Read the comment docs.

[mdc-web-bot]

All 541 screenshot tests passed for commit 3cc082a vs. master! 💯🎉

[mdc-web-bot]

All 541 screenshot tests passed for commit 925b568 vs. master! 💯🎉

[mdc-web-bot]

All 541 screenshot tests passed for commit 608afdd vs. master! 💯🎉

[mdc-web-bot]

All 541 screenshot tests passed for commit d66ae4a vs. master! 💯🎉