Aggregate association fix for Timescale >=0.20

[exterm]

Fixes https://github.com/timescale/timescaledb-ruby/issues/119, speculatively

This will fail on versions < 0.20. @jonatas any idea how to solve that problem? timescale >= 0.20 seems to be using a different metadata format than previous versions, so we may have to switch the approach based on extension version used.

[exterm]

Would love to see CI running on this as I can't execute the tests locally.

[jonatas]

Hi @exterm, I'm not working at Timescale, you may check some owners to enable it.