Consider multiple distribution files with REP-143

With REP-143 (https://github.com/ros-infrastructure/rep/pull/87) there can be multiple distribution files, so bloom with try to use the new api in rosdistro (https://github.com/ros-infrastructure/rosdistro/pull/45) to get a list of the distribution files and then take the last one. Otherwise it will fall back to the current rosdistro API, so this change is backwards compatible.