Solve HashDiff gem conflicts

The HashDiff constant used by this gem conflicts
with another gem of a similar name.
As of version 1.0 the HashDiff constant will
be completely removed and replaced by Hashdiff.

For more information see liufengyun/hashdiff#45.

Gemfile

@@ -52,6 +52,9 @@ gem 'wombat', '~> 2.8'
 gem 'flipflop', '~> 2.6'
 gem 'splitclient-rb', '~> 6.4'
 
+# Hashdiff is a ruby library to compute the smallest difference between two hashes.
+gem 'hashdiff', '~> 1.0'
+
 group :development, :test do
   # Call 'byebug' anywhere in the code to stop execution and get a debugger console
   gem 'byebug', platforms: %i[mri mingw x64_mingw]

Gemfile.lock

@@ -173,7 +173,7 @@ GEM
     has_scope (0.7.2)
       actionpack (>= 4.1)
       activesupport (>= 4.1)
-    hashdiff (0.4.0)
+    hashdiff (1.0.0)
     http-cookie (1.0.3)
       domain_name (~> 0.5)
     i18n (1.6.0)
@@ -466,6 +466,7 @@ DEPENDENCIES
   foreman
   geocoder (~> 1.5, >= 1.5.1)
   govuk-lint
+  hashdiff (~> 1.0)
   kaminari (~> 1.1, >= 1.1.1)
   listen (>= 3.0.5, < 3.2)
   logglier (~> 0.5)