doc: bump required version of PostgreSQL to 9.5+
doc: bump required version of PostgreSQL to 9.5+

The current BeeeOn Server is being tested mostly on PostgreSQL versions
10 and 9.5.

Signed-off-by: Jan Viktorin <3eab6ac54c005c4408f608556a6663b7f2abede7@fit.vutbr.cz>

BeeeOn/server,BeeeOn/server,BeeeOn/server,BeeeOn/server