Update GABA reversal potential with data from Seja 2012.

Reference: Seja, P.; Schonewille, M.; Spitzmaul, G.; Badura, A.;
Klein, I.; Rudhard, Y.; Wisden, W.; Hubner, C. A.; De Zeeuw, C. I. &
Jentsch, T. J. Raising cytosolic Cl- in cerebellar granule cells
affects their excitability and vestibulo-ocular learning EMBO J,
European Molecular Biology Organization, 2012, 31, 1217-1230