Memoize the Keeps::Helpers::Milestones#current_milestone method

Signed-off-by: Rémy Coutable <remy@rymai.me>

keeps/helpers/milestones.rb

@@ -18,8 +18,10 @@ def past_milestone(milestones_ago:)
       private
 
       def current_milestone
-        milestone = File.read(File.expand_path('../../VERSION', __dir__))
-        milestone.gsub(/^(\d+\.\d+).*$/, '\1').chomp
+        @current_milestone ||=
+          File.read(File.expand_path('../../VERSION', __dir__))
+          .gsub(/^(\d+\.\d+).*$/, '\1')
+          .chomp
       end
 
       def current_milestone_index