Handle Ensembl's release 75 refactoring of the Compara and Variation schemas

[FIXED] The Ensembl Compara changelog indicates "Split member into seq_member
     and gene_member + members depend on dnafrags". We now use the gene_member
     table. The change is backwards compatible.
[FIXED] The Variation.allele table no longer has a sample_id column. This is
    replaced by a population_id column.