Greg Kroah-Hartman, has announced that Kernel 4.1 will become the LTS version of 2015, meaning that it will still receive updates, despite the fact that newer Kernel 4.x versions will be released.
For now, Kernel 4.1 available only as a RC (Release Candidate) version, the stable version being scheduled for release in the few next weeks.
If you want to install and test Kernel 4.1 RC6 on your Ubuntu 15.04 system, follow the instructions from THIS ARTICLE.
Also, starting with Kernel 4.0, the Linux Kernel contains the Live Patching feature, which permits the users to upgrade the kernel without having to restart the system.