Spire
Once you have jailbroken your device you need to install Spire from your recently installed Cydia app. Open Cydia and go to Search, type in Spire and open the first result, tap Install and wait for  the installation. To complete. Be aware that the download takes up 100MB of space, the installation will take quite a while depending on your internet connection. Once the download is finished and the installation is complete, tap the Reboot icon to restart your device. Once restarted, go to Settings>General>Siri and toggle the switch to enable Siri.