Delete the call to __precompile__ and the warning will go away. If you want to continue supporting both 0.6 and 0.7/1.0 without warnings, you can do something like
VERSION < v"0.7-" && __precompile__()
FemtoCleaner will automatically remove such a statement once your package declares that it only support Julia versions ≥ 0.7.
For those watching this thread: removing the __precompile__() statement (or making it conditional based on VERSION) is no longer necessary now that https://github.com/JuliaLang/julia/pull/28459 has been merged.