CI and deployment with CircleCI

[dwwoelfel]

Per #2563 (comment), let's get bucklescript running on modern CI!

I can set up the Circle config for bucklescript. It would help a lot to know the following:

1. What are the test commands that need to be run
2. What environments do you want to test with (versions of node, different versions of ocaml?)

[chenglou]

cc @bobzhang moving off Travis should solve a bunch of issues, including the slow build in general. I don't think it's necessarily caused by our side. Even an npm install seems slow on Travis for BS

[bobzhang]

this is all the config I did for travis: https://github.com/BuckleScript/bucklescript/blob/master/.travis.yml

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.