Correct Firefox Beta 29.0b1 checksum

The sha256 checksum does not match.
```
» brew cask install firefox-beta
==> Downloading https://download.mozilla.org/?product=firefox-29.0b1&os=osx&lang=en-US
######################################################################## 100.0%
Error: SHA2 mismatch
Expected: 5f13ff1fdae28e09c50ebaf915802e275918ca32e411fcd7a61f466efe68a042
Actual: f2894b188e1738360f269c1190370c3880c6886306486ba7da626ab2eeef6368
Archive: /Library/Caches/Homebrew/firefox-beta-29.0b1.0b1&os=osx&lang=en-US
To retry an incomplete download, remove the file above.
```

Unfortunately, Mozilla does not provide sha256's to compare to. But I did compare my download to the [sha512 provided by Mozilla](http://ftp.mozilla.org/pub/mozilla.org/firefox/releases/29.0b1/SHA512SUMS).

```
~/Downloads » shasum -a 256 "Firefox 29.0b1.dmg"
f2894b188e1738360f269c1190370c3880c6886306486ba7da626ab2eeef6368  Firefox 29.0b1.dmg
~/Downloads » shasum -a 512 "Firefox 29.0b1.dmg"
ca6dccf4adfc7d3dfef2d0e5959769953860ea064a4cf57b3cadfd00ab4f0bd8765fa38fa453d4701b11c1a45f3e044bddf9b0d560cd86c7734aa630bee035af  Firefox 29.0b1.dmg
```