Use Travis CI for continuous integration builds per-PR

@DSASanFrancisco/portal-members

Sorry, @scudmissile, I didn't like how Circle CI could not be disabled for branches that don't have a .circle.yml file so I dug into the differences between Circle CI and Travis CI. I think Travis better supports the open-source community, provides what we need with a simple interface, and has some advantages, like S3 caching, that might be useful for UI builds. I'm open to switching to Circle CI if there are any big advantages in build time or configurability, but for now, I'd say let's go with the company that best serves the community.

This would fulfill the same purpose as https://github.com/DSASanFrancisco/membership_api/pull/30