Migrate from Travis to CircleCI for tests.

[pglombardo]

We love Travis and it's treated us well over the years but we are switching to CircleCI for the following reasons:

1. we already have a paid CircleCI account for another project (dedicated build resources)
2. we've found CircleCI to be much faster probably because of above
3. CircleCI integrations and workflows appear cleaner and more straight forward
4. Travis has had it's fair amount time consuming service outages (we'll see with CircleCI though)