Change POIs displayed at z15 and z16 as discussed on #689

makes #70 less severe or maybe even fixed
follow CartoCSS Style Guidelines
"Always specify zoom levels as either >= or < . Don't use = or =< or >"
Merge branch 'change_POIs_appearing_on_z15_and_z16' of https://github.com/mkoniecz/openstreetmap-carto into mkoniecz-change_POIs_appearing_on_z15_and_z16

Conflicts:
	amenity-points.mss
Merge branch 'mkoniecz-change_POIs_appearing_on_z15_and_z16'