Allow timescale to be a non-primary rails db

[segiddins]

Enables rubygems.org to use timescale as a secondary db only for gem downloads: https://github.com/rubygems/rfcs/pull/48

Consumed in https://github.com/rubygems/rubygems.org/pull/3560

[santiagodoldan]

@segiddins Thank you so much for your contribution, it's awesome to hear that rubygems will use the gem 🚀

[santiagodoldan]

@segiddins One last thing, could you follow conventional commits specs -> https://github.com/crunchloop/timescaledb-rails/blob/main/CONTRIBUTING.md#pull-requests?

[segiddins]

@santiagodoldan feel free to change the commit message when squash-merging

[santiagodoldan]

@segiddins Sure, that works too, let me know about my last 2 comments and I will approve your PR when they are done, I'll push a new version to rubygems if everything works as expected 🚀

[santiagodoldan]

@segiddins Hey, let me know if my comments make sense to you, so I can resolve them and merge your changes 🚀

[santiagodoldan]

@segiddins Looks like ruby2_keywords was introduced in ruby 2.7

[santiagodoldan]

@segiddins Thanks for your contribution 💯