Update beaver from v0.3.3 to v0.4.0

* beaver v0.3.3(cbce378)...v0.4.0(61aebb1) (10 commits)
  > dlang/make: Rely on MakD to properly merge cov reports
  > dlang/codecov: Improve detection of lack of reports
  > dlang/codecov: Remove wrong change of directory
  > Add a new general-purpose codecov command
  > beaver: Fail if the requested command doesn't exist
  (...)