Fix bug #15: Errno:EACCES on Windows when trying to compile identical file (on watch)

Per https://prototype.lighthouseapp.com/projects/8888/tickets/16-permission-denied-building-via-sprockets-on-windows you can't do File.utime on an open file in Windows.  Changed the reading of the existing file to IO.read() which will close the file on completion of the read.

lib/compass/actions.rb

@@ -33,7 +33,7 @@ module Compass
       options ||= self.options if self.respond_to?(:options)
       skip_write = options[:dry_run]
       if File.exists?(file_name)
-        existing_contents = File.new(file_name).read
+        existing_contents = IO.read(file_name)
         if existing_contents == contents
           logger.record :identical, basename(file_name)
           skip_write = true
@@ -105,4 +105,4 @@ module Compass
     end
 
   end
-end
+end