Update to latest cartodb-postgresql version

Fixes potential infinite loop in CDB_QueryStatements
CDB-1797

Includes yet another rewrite of load_cartodb_functions to correctly
load the now-required plpythonu extension
Document cartodb-postgresql and postgis-as-extension requirements
Merge branch 'CDB-2936' of https://github.com/CartoDB/cartodb into CDB-3061

Conflicts:
	lib/sql