Switch to CircleCI

[cwillisf]

Switch from Travis CI to Circle CI.

There are a handful of build-related scripts in the travis/ directory, and some of them expect the environment variables that Travis CI provides. Rather than modify those scripts, I chose to set the variables they expect from their CircleCI counterparts when they're called from .circleci/config.yml. For example:

TRAVIS_BRANCH="${CIRCLECI_BRANCH}" travis/setup-git.sh

I chose this approach because upstream still uses Travis. If they alter these build scripts, keeping them "clean" will make it easier to merge from upstream in the future.

The rest of the config attempts to match the previous behavior, with the exception that I added package-lock.json and use npm ci through the node/install-packages command from the node orb.

[aoneill01]

Looks good! What do you think about updating the build badge in the README to point to CircleCI? Maybe not worth it if we are trying to keep this as close to the original fork as possible.

[cwillisf]

Ah, I missed the badge -- thank you!

Hmm... I see several links in there that would probably be better if they pointed at our version, or were removed entirely. The CI and NPM links in particular feel a bit like lying, though. Maybe I'll add a bit at the top and replace that one line. That shouldn't complicate merges too much...