Another option for jailbreak development without a Mac is to compile your code on the iPhone itself over SSH! You can use Cydia to install an OpenSSH server on your iPhone. Then, log in using ssh from the Ubuntu terminal, or PuTTY on Windows.
Once SSH is up-and-running, you'll need to install a toolchain on your device. Try the "iOS Toolchain" package from the BigBoss repository in Cydia.
You should then be able to install Theos onto the device, though you may need to find iOS SDK headers in order to compile against iOS frameworks. The official headers can be found as part of Xcode (in a folder called iPhoneOS7.1.sdk), but you could also potentially use dumped headers.
Here is a brief tutorial for how to do this.
Additionally, it's possible to set up an iOS development environment on Windows. See this guide!