change circleci to build PR + master branch before merge

This is for Theo L. Currently our CircleCI workflow is automatically triggered for each new pull request. However, the master build is only triggered upon merging the PR. This has the downside that master builds can fail even though the PR branch runs ok. Deliverable: * change the workflow configuration in github and/or circleci so that for each PR the build is triggered for both the PR branch as well as the master branch References: * [login to view URL] * [login to view URL] * [login to view URL]