Relax plailude dependency to <0.4

tempuhs-server is planned to require plailude 0.3. tempuhs is already
compatible with plailude 0.3, so only the cabal file needs changing.

Signed-off-by: Stian Ellingsen    <stian@plaimi.net>
Reviewed-by:   Alexander Berntsen <alexander@plaimi.net>
Signed-off-by: Alexander Berntsen <alexander@plaimi.net>