Add GitHub Actions CI and an ActiveSupport compatibility matrix

[nnc]

What

Replaces the (effectively empty) Travis config with a GitHub Actions workflow that runs the unit suite across a Ruby × ActiveSupport matrix.

• CI (.github/workflows/ci.yml): runs bundle exec rake test (unit-only; the suite stubs all Xero HTTP, so it needs no credentials or secrets).

• Matrix — a minimal covering set: each supported Ruby once and each supported ActiveSupport once, pinned per-cell via gemfiles/ + BUNDLE_GEMFILE:

  Ruby	ActiveSupport
  3.1	7.0
  3.2	7.1
  3.3	7.2
  3.4	8.0
  4.0	8.1
  ruby-head	Rails main (experimental, non-blocking)

• Supported versions: sets required_ruby_version >= 3.1 and raises the activesupport floor to >= 7.0.

• README: adds CI and gem-version badges.

• Removes .travis.yml.

Notable: a load-time crash on some ActiveSupport versions

Building the matrix surfaced a real bug. lib/xeroizer.rb cherry-picks ActiveSupport core extensions (e.g. active_support/core_ext/array) without first loading active_support. Some AS versions emit a deprecation at load time from those files:

• AS 7.1 — array/conversions calls ActiveSupport.deprecator
• AS 8.2 (currently Rails main) — time/conversions references ActiveSupport::Deprecation

…both undefined unless the deprecation framework is already loaded, so require 'xeroizer' raises NoMethodError / NameError.

The fix loads just the deprecation framework (not all of ActiveSupport):

require 'active_support/deprecation'
begin
  require 'active_support/deprecator' # added in AS 7.1; absent on 7.0
rescue LoadError
end

The rescue LoadError is necessary: active_support/deprecator only exists from AS 7.1, and LoadError isn't a StandardError (so a bare rescue modifier wouldn't catch it).

Testing

All cells pass — 191 tests, 0 failures — including ruby-head + Rails main. The change touches no test files.