fix issue with missing dub.selections.json

- the dub.selections.json file was no longer generated
  for packages without dependencies

- this lead to an error when checking for the timestamp
  of the selections file later on

- an alternative fix would be to ignore the timestamp
  when no selections file exists
Merge pull request #320 from MartinNowak/fixNoDepBuild

fix issue with missing dub.selections.json