Update docs for Berkeley DB 5.3 or greater

Summary:
D1330 changed the requirements for Berkeley DB.
This updates the docs to reflect the new requirement.

Test Plan:
Follow the new instructions, check that it builds.
I tested on Ubuntu.
If possible, others could check: Debian, OSX, FreeBSD, and Open BSD.

Reviewers: deadalnix, schancel, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1409