* Use containers for testing again
Disable chrome security sandbox due to travis-ci/travis-ci#8836
Remove node module caching with the removal of package lock
* flag is "no-sandbox"
* Add back npm cache
* Use sudo to fix chrome testing
There is a bug with the chrome sandbox in the Travis CI container
system, which is fixed by using `sudo: required`.
Related to travis-ci/travis-ci#8836
* reduce cognitive overhead of the test runner
* remove testing polyfilled Custom Elements with native ShadowDOM
* Use headless chrome and firefox
Use MacOS 10.13 for Safari 11
* disable testing CSS Custom Property Shim
* move sauce tests into script for travis_wait compat
