Edit: Forge versions 59+ have re-added the missing interfaces, so those are compatible again.
Forge updated to 1.4.1 (.58) and unexpectedly removed an interface required by BuildCraft in the process. That will cause BuildCraft to crash with the new “recommended” build of Forge.
We won’t be pushing a hotfix for that but continue on our normal release schedule. For now you will have to use the last working build 188.8.131.52 available here: