Relax base64 dependency to allow 0.3.x

[romanoff]

Summary

Relaxes the base64 runtime dependency from ~> 0.2.0 to >= 0.2, < 0.4.

Motivation

The exact ~> 0.2.0 pin (added in #920 to accommodate the gem's removal from the Ruby 3.4 stdlib) makes this client uninstallable alongside gems that require base64 ~> 0.3.0 — for example travel_time >= 0.8.0. Bundler cannot resolve a bundle containing both:

Because recurly >= 4.59.0 depends on base64 ~> 0.2.0
  and travel_time >= 0.8.0 depends on base64 ~> 0.3.0,
  recurly >= 4.59.0 is incompatible with travel_time >= 0.8.0.

Safety

• The client's only use of the gem is a single Base64.encode64 call for the Basic auth header in lib/recurly/client.rb, whose behavior is unchanged in base64 0.3.x.
• The full test suite passes against base64 0.3.0: 920 examples, 0 failures.
• The same relaxation was made by fog-aws (>= 0.2, < 0.4) for the same reason.