sRNA:add mirge to memory usage configuration.
sRNA:add mirge to memory usage configuration.

thanks to @mshadbolt
github.com/bcbio/bcbio-nextgen/issues/2379

lbeltrame/bcbio-nextgen,a113n/bcbio-nextgen,vladsaveliev/bcbio-nextgen,chapmanb/bcbio-nextgen,vladsaveliev/bcbio-nextgen,a113n/bcbio-nextgen,chapmanb/bcbio-nextgen,chapmanb/bcbio-nextgen,vladsaveliev/bcbio-nextgen,lbeltrame/bcbio-nextgen,lbeltrame/bcbio-nextgen,a113n/bcbio-nextgen