updating genomes to the latest Ensembl 51 release

[COTRASIF]

Ensembl released version 51 on the 18th of November.

We have just initiated the genomes update procedure.
Unless there were API/naming changes in this release, update should be
finished shortly.

Genomes being updated will be transiently unavailable.

[Bogdan]

Update went smoothly.

Also,

+ Equus caballus, horse (added: promoters of the protein-coding genes
with experimentally known transcripts, and full orthology mappings)