Update to ActiveRecord 5 for higher ruby version compat

[morgvanny]

ActiveRecord 4 no longer works with Ruby 2.7.1, but version 6 seems to not work properly with Sinatra and migrations won't run with it, so version 5 seems like a happy medium to stick with for a while.

[adisciple50]

by using gem 'bigdecimal', '1.3.5' we can have 6! but i dont know what we would lose. tests needed.

[adisciple50]

put gem 'bigdecimal', '1.3.5' in the boilerplate Gemfile.

[adisciple50]

it works, but i have not tested this!

the early bigdecimal does not warn of bigdecimal().new. i found this on some arcane forum tomb of a page.

[morgvanny]

Ah yeah I got version 6 working at some point after this, and never came back to it

[JuanVqz]

I tested this PR, it is working as expected. thank you!