Remove images of the same name before pulling a new one

Or you'll end up with untagged dupes.

lib/mrsk/commands/builder.rb

@@ -1,5 +1,5 @@
 class Mrsk::Commands::Builder < Mrsk::Commands::Base
-  delegate :create, :remove, :push, :pull, :info, to: :target
+  delegate :create, :remove, :push, :clean, :pull, :info, to: :target
 
   def name
     target.class.to_s.remove("Mrsk::Commands::Builder::").underscore

lib/mrsk/commands/builder/base.rb

@@ -1,6 +1,10 @@
 class Mrsk::Commands::Builder::Base < Mrsk::Commands::Base
   delegate :argumentize, to: Mrsk::Utils
 
+  def clean
+    docker :image, :rm, "--force", config.absolute_image
+  end
+
   def pull
     docker :pull, config.absolute_image
   end