gecko.py: constrain_pool() not working with cobra 0.17.1 version

[sousamd]

This line (213) on gecko.py

protein_pool = self.metabolites.get_by_id('prot_{}_c'.format(protein_id))

should be

protein_pool = self.metabolites.get_by_id('prot_{}[c]'.format(protein_id))

This is not a problem with cobra 0.15.1. Don't know about 0.16.