Update minimum rake version to satisfy Dependabot

https://github.com/steveklabnik/request_store/security/dependabot/2

Gemfile

@@ -7,8 +7,7 @@ case Gem::Version.new(RUBY_VERSION.dup)
 when ->(ruby_version) { ruby_version >= Gem::Version.new('2.2.0') }
   gem 'rake', '~> 13'
 when ->(ruby_version) { ruby_version >= Gem::Version.new('2.0.0') }
-  gem 'rake', '~> 12'
+  gem 'rake', '~> 12.3.3'
 else
   gem 'rake', '~> 11'
 end
-