transition from TravisCI to CircleCI #1860

[stephen]

To Dos

• Test against multiple Node versions (4, 6, 7)
• Exit early if one Node version fails (don't keep testing on the other Node versions)
• OS X (requires reaching out to Circle)
• Deploy to npm after tagged build (maybe)

[/stephen]

Though we trust that the master branch is building successfully, the build failing on TravisCI with resulting red badge on project level README is not a good experience for our customers.

Actions: move from TravisCI to CircleCI

Other options for discussion:

1. Change default branch from master; leave master for contributions and ongoing dev.
2. ???

@jgeewax @omaray @jmuk @stephenplusplus @callmehiphop