fix this bug https://trac.openstreetmap.org/ticket/2832

Changed from almost never used amenity=picnic_site to tourism=picnic_site, and also added leisure=picnic_table with same symbol.
Update project.mml
corrected
Merge branch 'patch-1' of https://github.com/altoing/openstreetmap-carto into altoing-patch-1
Remove unnecessary datasource parameters.