Update firefox-nightly.rb

Fix for Error SHA2 mismatch.

```
$ brew cask install firefox-nightly
==> Downloading http://ftp.mozilla.org/pub/mozilla.org/firefox/nightly/latest-trunk/firefox-31.0a1.en-US.mac.dmg 
######################################################################## 100.0%
Error: SHA2 mismatch
Expected: ad6ca5707ef3c1e478d5f04be4f51eeca6599e94ec9609860e028f271cfafe3e
Actual: 28adf442f339ac3272d1ad14d657e902fbe1e7d6ebb4e007103513aca5a1990c
Archive: /Library/Caches/Homebrew/firefox-nightly-31.0a1.dmg
```

```
$ brew cask audit firefox-nightly --download
==> Downloading http://ftp.mozilla.org/pub/mozilla.org/firefox/nightly/latest-trunk/firefox-31.0a1.en-US.mac.dmg
######################################################################## 100.0%
audit for firefox-nightly: passed
```