Change tuple metrics to double

We are experiencing some errors on system.log due to overflow of tuple values on production postgresql server like:

/usr/sbin/gmetad[1027]: RRD_update (/var/lib/ganglia/rrds/**/__SummaryInfo__/pg_tup_returned.rrd): /var/lib/ganglia/rrds/**/__SummaryInfo__/pg_tup_returned.rrd: not a simple unsigned integer: '-945436630'