Correctly override mutant rake task

Rakefile

@@ -1,24 +1,23 @@
 require 'devtools'
 Devtools.init_rake_tasks
 
-Rake::TaskManager.class_eval do
-  def remove_task(task_name)
-    @tasks.delete(task_name.to_s)
+class Rake::Task
+  def overwrite(&block)
+    @actions.clear
+    enhance(&block)
   end
 end
 
+Rake.application.load_imports
 
-namespace :metrics do
-  desc 'Run mutant'
-  task :mutant => :coverage do
-    project = Devtools.project
-    require File.expand_path('../spec/support/zombie.rb', __FILE__)
-    Zombie.setup
-    status = Zombie::CLI.run(%W(::Mutant --rspec-dm2))
-    if status.nonzero?
-      $stderr.puts 'Zombie task is not successful'
-      $stderr.puts 'Not fatal at this point of development, will be fixed before release of 0.3.0'
-    end
-    Kernel.exit(0) # FIXME: Prevent devtools mutant task
+Rake::Task['metrics:mutant'].overwrite do
+  project = Devtools.project
+  require File.expand_path('../spec/support/zombie.rb', __FILE__)
+  Zombie.setup
+  status = Zombie::CLI.run(%W(::Mutant --rspec-dm2))
+  if status.nonzero?
+    $stderr.puts 'Zombie task is not successful'
+    $stderr.puts 'Not fatal at this point of development, will be fixed before release of 0.3.0'
   end
+  Kernel.exit(0) # FIXME: Prevent devtools mutant task
 end