CircleCI integration for forked pull requests

[mdrose]

Description

The CircleCI integration for this repository gets stuck in an "expected" status, as evidenced by pull requests I've created recently (#145 and #146).

[liliakai]

I believe the Circle project is configured not to build forked pull requests for security reasons.

[liliakai]

Here's what Circle has to say about security and forked PR's: https://circleci.com/docs/2.0/oss/

It seems to me that we could safely enable Build forked Pull Requests without passing secrets to them.

Perhaps @conorsch or @garrettr can comment on why this wasn't enabled previously, or are able and willing to enable it in the Circle project settings?

In the meantime I'm pushing PRs to local branches (after manual review) in order to trigger the builds.