Ensure diskimages are overwritten when building

Rakefile

@@ -89,8 +89,7 @@ namespace :build do
 
     desc "Build a disk image for the iOS static framework"
     task :diskimage => :framework do
-      FileUtils.mkdir_p("pkg")
-      system("hdiutil create -srcfolder #{BUILD_DIR}/Release pkg/LRResty-iOS-#{PACKAGE_SUFFIX}.dmg")
+      create_disk_image("pkg", "#{BUILD_DIR}/Release", "LRResty-iOS-#{PACKAGE_SUFFIX}.dmg")
     end
   end
 
@@ -100,8 +99,7 @@ namespace :build do
 
     desc "Build a disk image for the Mac framework"
     task :diskimage => :framework do
-      FileUtils.mkdir_p("pkg")
-      system("hdiutil create -srcfolder #{BUILD_DIR}/Release pkg/LRResty-Mac-#{PACKAGE_SUFFIX}.dmg")
+      create_disk_image("pkg", "#{BUILD_DIR}/Release", "LRResty-Mac-#{PACKAGE_SUFFIX}.dmg")
     end
   end
 
@@ -142,6 +140,11 @@ namespace :build do
     end
   end
 
+  def create_disk_image(destination, source, output_file)
+    FileUtils.mkdir_p(destination)
+    system("hdiutil create -srcfolder #{source} #{destination}/#{output_file} -ov")
+  end
+
   def prompt_for_password
     fetch_password_from_keychain(GITHUB_USER) || ask("Enter Github password (#{GITHUB_USER}):", String) {|q| q.echo = false }.tap do |password|
       store_password_in_keychain(GITHUB_USER, password)