Suppress a Ruby's warning when using Ruby 2.6.0+

This PR suppresses the following warning that `deep_merge` method and
`deep_update` method are defined twice when using Ruby 2.6.0+.

```console
% bundle exec rake
(snip)

/Users/koic/src/github.com/hahie/hashie/lib/hashie/mash.rb:226: warning:
method redefined; discarding old deep_merge
/Users/koic/src/github.com/hahie/hashie/lib/hashie/mash.rb:212: warning:
previous definition of deep_merge was here
/Users/koic/src/github.com/hahie/hashie/lib/hashie/mash.rb:232: warning:
method redefined; discarding old deep_update
/Users/koic/src/github.com/hahie/hashie/lib/hashie/mash.rb:218: warning:
previous definition of deep_update was here
```

1 files changed, 5 insertions, 1 deletions

diff --git a/lib/hashie/extensions/ruby_version_check.rb b/lib/hashie/extensions/ruby_version_check.rb
index bcd3436..b287230 100644
--- a/lib/hashie/extensions/ruby_version_check.rb
+++ b/lib/hashie/extensions/ruby_version_check.rb
@@ -9,7 +9,11 @@ module Hashie
 
       module ClassMethods
         def with_minimum_ruby(version)
-          yield if RubyVersion.new(RUBY_VERSION) >= RubyVersion.new(version)
+          yield if with_minimum_ruby?(version)
+        end
+
+        def with_minimum_ruby?(version)
+          RubyVersion.new(RUBY_VERSION) >= RubyVersion.new(version)
         end
       end
     end