Use merge pull/checkout for pull requests (#802)
Use merge pull/checkout for pull requests (#802)

Unfortunately CIRCLE_PR_NUMBER does not work (outside forks?), see:
- https://discuss.circleci.com/t/circle-pr-number-missing-from-environment-variables/3745/4
- https://discuss.circleci.com/t/pull-request-number/972/12

Thus using `CIRCLE_PULL_REQUEST` to extract the PR number.
aeternity/epoch,aeternity/epoch,aeternity/epoch,aeternity/epoch