Skip MINT if the tRNA files are not installed.
Skip MINT if the tRNA files are not installed.

Thanks @lpantano for suggesting the fix and thanks to @huipan1973
for opening up the issue.

Closes #2905.

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