Are you a registered iOS Developer? Downloading beta versions of iOS without being a registered iOS developer is a form of piracy and could result in an iPhone that becomes useless because it cannot be activated.
There is currently no jailbreak for iOS 6.1.3 and the evasi0n team has indicated that they won't be developing one as they don't want to burn an exploit that they might be able to use in developing n iOS 7 jailbreak.