Currently the only emulator for testing iOS applications is build for MacOS.
There is a work in process to bring an emulation software to linux which you might want to take a look at...but judging by the development status it'll be a while till it's usable.
http://www.iemu.org/index.php/Main_Page

The long and complicated way of achieving your goal would be running MacOs in a virtualbox on ubuntu and run the official emulator in there.

Currently the only emulator for testing iOS applications is build for MacOS.

There is a work in process to bring an emulation software to linux which you might want to take a look at...but judging by the development status it'll be a while till it's usable.
http://www.iemu.org/index.php/Main_Page

EDIT: This is outdated information. The project was discontinued some time in 2013.

Your options at this time would be to try and install a hackintosh instance in virtualbox or use something like Appetize.io, if you have a bundled app that you want to try out.

For real productive development work I'm afraid you won't get around buying Apple hardware.