Fix compatibility with older versions of Ruby (< 1.9.2).

Currently "compare_images" tasks fail when using Ruby < 1.9.3, because File.write is only available in Ruby 1.9.3 and above:

```bash
rake aborted!
undefined method `write' for File:Class
/wraith/Rakefile:72
Tasks: TOP => default => compare_images
(See full trace by running task with --trace
```

This pull requests fixes that and uses a method which is also available in older versions.

Reference:

* Ruby 1.9.3 http://ruby-doc.org/core-1.9.3/IO.html#method-c-write
* Ruby 1.9.3 http://ruby-doc.org/core-1.9.2/IO.html#method-c-write (there is no IO.write method)