[v4] Removing old Rails checks

[GBH]

Since we don't care about Rails 3 and 4 we don't need those checks (not like any of that stuff was tested anyways)

[bootstrap-ruby-bot]

	1 Warning
⚠️	Please update CHANGELOG.md with a description of your changes. If this PR is not a user-facing change (e.g. just refactoring), you can disregard this.

Here's an example of a CHANGELOG.md entry (place it immediately under the * Your contribution here! line):

* [#381](https://github.com/bootstrap-ruby/rails-bootstrap-forms/pull/381): [v4] Removing old Rails checks - [@GBH](https://github.com/GBH).

Generated by 🚫 Danger

[lcreid]

Looks good to me. I was debating whether to ask for a CHANGELOG entry, but I'll let this one go without.

[mattbrictson]

Thanks, nice cleanup 🙌