Minimum FDB version 5.11

[jhardenberg]

This small fix changes the minimum required FDB version to 5.11.0.
In fact, without this fix, due to the API changes, pyfdb would complain if confronted with a 5.10 (or less) FDB binary library, with messages like

function/symbol 'fdb_new_splitkey' not found in library '/usr/local/lib/libfdb5.so': /usr/local/lib/libfdb5.so: undefined symbol: fdb_new_splitkey

Error retrieving attribute fdb_new_splitkey from library

but still not raise a proper error.

[FussyDuck]

All committers have signed the CLA.