Pipelines for pull requests from forks

It would be great if the pipeline feature could build any PRs submitted from forks of the project. For example, when I submit a PR from my fork here:

https://bitbucket.org/pysph/pysph/pull-requests/259/

the pipeline build does not run. On branches that are pushed to the main project, things work fine though but this is not useful when someone without write access to the main repository pushes a PR from their fork. I apologize if this has already been requested.