VEP: replaced ExAC with GnomAD
VEP: replaced ExAC with GnomAD

The `--af_exac` flag had been deprecated in favor of the `--af_gnomad` flags, which includes annotations of all GnomAD exome frequencies. Using the `--af_exac` flag breaks VEP, even though it's still referenced in the docs.
see ensembl/ensembl-vep#76
vladsaveliev/bcbio-nextgen,a113n/bcbio-nextgen,lbeltrame/bcbio-nextgen,vladsaveliev/bcbio-nextgen,lbeltrame/bcbio-nextgen,vladsaveliev/bcbio-nextgen,chapmanb/bcbio-nextgen,a113n/bcbio-nextgen,biocyberman/bcbio-nextgen,lbeltrame/bcbio-nextgen,biocyberman/bcbio-nextgen,chapmanb/bcbio-nextgen,chapmanb/bcbio-nextgen,biocyberman/bcbio-nextgen,a113n/bcbio-nextgen