Fix sha256 in 'firefox-aurora.rb'

SHA2 mismatch error ...
```bash
==> Downloading https://ftp.mozilla.org/pub/mozilla.org/firefox/nightly/latest-mozilla-aurora/firefox-35.0a2.en-US.mac.dmg
######################################################################## 100.0%
==> Note: running "brew update" may fix checksum errors
Error: SHA2 mismatch
Expected: 36e201fe9772d89c68f6555a78748657748da2a902b8334d11bbd2b35f6c720f
Actual: c27899ebdb9d3b44ccac8ad904a03b421b65208e66937c62d795aab1d0014999
Archive: /Library/Caches/Homebrew/firefox-aurora-35.0a2.dmg
To retry an incomplete download, remove the file above.
```

I also checkt the downloaded file against the sha512 from https://ftp.mozilla.org/pub/mozilla.org/firefox/nightly/latest-mozilla-aurora/firefox-35.0a2.en-US.mac.checksums