uses Digest::MD5::hexdigest instead of md5sum for better compatibility

Rather than rely on md5sum, I changed a single line to use Ruby's built-in md5 digest method.

On OSX (at least my 2011 MBA), md5sum is not installed. This is required to run `rake integrity_yaml`, which is in turn required to add any PDFs to the test suite.

To ensure that this works, I renamed the `integrity.yml` file to `old_integrity.yml`, re-ran `rake integrity_yaml` and compared md5 checksums of both files (how meta!):

```sh
$ md5 spec/integrity.yml
MD5 (spec/integrity.yml) = ef93ad82b30e44141a7cd90be1605228
$ md5 spec/old_integrity.yml
MD5 (spec/old_integrity.yml) = ef93ad82b30e44141a7cd90be1605228
```