Upgrade to PostgreSQL 11

[artur-intech]

• Make sure pg gem is compatible
• Check generate_zonefile function

[teadur]

@artur-beljajev As of our next deployment platform has psql 11 client libraries we decided that next db upgrade will be to version 11.

We can reconsider this if there is some functionality we are missing in version 11 and would be present in version 12.

If not I propose that we change the issue to upgrade to PostgreSQL 11

[artur-intech]

Totally fine, title updated.