Migrate CI from Travis CI to CircleCI

[yhatt]

Migrate CI from Travis CI to CircleCI. This PR is a feedback from @marp-team/marp-core.

We feel that Travis CI build performance is not fast so much. Sometimes there was a time lag. The lagged build triggers with about a minute lag after pushing.

So we have tried using CircleCI on @marp-team/marp-core. The code base does not much bigger, but we satisfied with the great build performance on CircleCI, that only spends about a minute without lags for building!

One of the cons is that the configuration becomes complicated, but we receive many pros instead:

• CircleCI's build artifacts can host the outputted coverage HTML per builds.
• CircleCI's workflow can be composed of several jobs. So in future, we might be able to automate publishing to npm by setting CI jobs.
• The owner of marp-team is Japanese, so the existence of CircleCI Japan means that we could receive better support about management of marp-team.