Only add base64 to Gemfile for Ruby >= v3.3.0

The base64 gem does not support Ruby v2.2 and so the previous commit [1]
was causing a CI build failure [2].

[1]: 8c49314
[2]: https://app.circleci.com/pipelines/github/freerange/mocha/695/workflows/4c622995-5468-4cfc-a3cf-405fecb39cb0/jobs/10161

Gemfile

@@ -2,7 +2,6 @@ source 'https://rubygems.org'
 
 gemspec
 
-gem 'base64'
 gem 'introspection', '~> 0.0.1'
 gem 'jaro_winkler', '>= 1.5.5'
 gem 'minitest'
@@ -14,6 +13,11 @@ if RUBY_VERSION >= '3.1.0'
   gem 'psych', '< 4'
 end
 
+# Avoid base64 gem warning about it not being available in Ruby v3.4
+if RUBY_VERSION >= '3.3.0'
+  gem 'base64'
+end
+
 if RUBY_ENGINE == 'jruby'
   # Workaround for https://github.com/jruby/jruby/issues/8488
   gem 'jar-dependencies', '~> 0.4.1'