CircleCI build for PRs

[bquorning]

I noticed on #38 that the CircleCI build wasn’t running. Checking our Circle config I found that it was set to not build PRs from forked repositories. I’ve now enabled that setting, and I assume new pull requests (and new commits to existing PRs) will trigger a build.

/cc @koic

[koic]

Thank you for reviewing the setting. Let's see the next build result.