brew-create: don't break when given --set-version

Since 9f2782812cbaf2, specifying a version breaks with:

    Error: undefined method `detected_from_url?' for "1.2.2":String

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

Library/Homebrew/cmd/create.rb

@@ -84,7 +84,9 @@ class FormulaCreator
     else
       @path = Formula.path name
     end
-    if @version.nil?
+    if @version
+      @version = Version.new(@version)
+    else
       @version = Pathname.new(url).version
     end
   end