Specs on GitHub actions, BigDecimal dep. for Ruby 3.5, Redis gem update, require Ruby 3.1

[swiknaba]

👋 Thanks for taking over the repo from Skyscanner/historical-bank-ruby#4

Links to PRs are likely broken when we moved the fork, here our old fork with PRs: https://github.com/lh-innovationhub/historical-bank-ruby-old

TL;DR:

• better performance
• added BigDecimal to prepare for Ruby 3.5
• added GitHub Actions to at least run RSpec
• updated Syntax to be compatible with Redis 4 / 5 (gem version)
• dropped pinned versions for dev-dependencies (updated Faker & its syntax)
• dropped script to run redis-server locally (we assume you have redis running locally on the default port, if not, you can still pass a custom port) --> irrelevant for CI which spins up Redis using GitHub Action's Redis
• ⚠️ require Ruby 3.1 (anyone still on Ruby 2 can just use an old version)

Note: The redis gem version is super relaxed atm; we've tested this with v4 and v5. Ideally, we'd use, e.g., appraisal to run tests against both versions (or you pin this down to only one major version). -- just didn't have time for that yet.

Travis vs. GitHub actions: no preferences here, there was some hiccups with Travis a while ago so we just switched got GH actions. I see it works on your master branch, feel free to revert to Travis.

We assumed the project was dead for good so we weren't squeamish 😄

Let me know what you think; happy to improve :-)