Ubuntu: auto select current release (iovisor#2057)

auto select current release for ubuntu

INSTALL.md

@@ -58,7 +58,7 @@ The stable and the nightly packages are built for Ubuntu Xenial (16.04), Ubuntu
 
 ```bash
 sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 4052245BD4284CDD
-echo "deb https://repo.iovisor.org/apt/xenial xenial main" | sudo tee /etc/apt/sources.list.d/iovisor.list
+echo "deb https://repo.iovisor.org/apt/$(lsb_release -cs) $(lsb_release -cs) main" | sudo tee /etc/apt/sources.list.d/iovisor.list
 sudo apt-get update
 sudo apt-get install bcc-tools libbcc-examples linux-headers-$(uname -r)
 ```