Update xpdf to 3.04 for swftools

I discovered today that xpdf has updated to 3.04 when I was trying to install [swftools]. This updates the resource to the new version. I followed [the guide](https://github.com/Homebrew/homebrew/wiki/How-To-Open-a-Homebrew-Pull-Request-(and-get-it-merged)) and all tests passed.

[swftools]: www.swftools.org
Oops!