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, 14 insertions, 14 deletions

diff --git a/lib/hashie/mash.rb b/lib/hashie/mash.rb
index 194a6f8..96b24c9 100644
--- a/lib/hashie/mash.rb
+++ b/lib/hashie/mash.rb
@@ -207,20 +207,7 @@ module Hashie
     alias include? key?
     alias member? key?
 
-    # Performs a deep_update on a duplicate of the
-    # current mash.
-    def deep_merge(other_hash, &blk)
-      dup.deep_update(other_hash, &blk)
-    end
-
-    # Recursively merges this mash with the passed
-    # in hash, merging each hash in the hierarchy.
-    def deep_update(other_hash, &blk)
-      _deep_update(other_hash, &blk)
-      self
-    end
-
-    with_minimum_ruby('2.6.0') do
+    if with_minimum_ruby?('2.6.0')
       # Performs a deep_update on a duplicate of the
       # current mash.
       def deep_merge(*other_hashes, &blk)
@@ -235,6 +222,19 @@ module Hashie
         end
         self
       end
+    else
+      # Performs a deep_update on a duplicate of the
+      # current mash.
+      def deep_merge(other_hash, &blk)
+        dup.deep_update(other_hash, &blk)
+      end
+
+      # Recursively merges this mash with the passed
+      # in hash, merging each hash in the hierarchy.
+      def deep_update(other_hash, &blk)
+        _deep_update(other_hash, &blk)
+        self
+      end
     end
 
     # Alias these lexically so they get the correctly defined