Update info about Ubuntu Xenial in INSTALL.md
Update info about Ubuntu Xenial in INSTALL.md

Update info about installing stable packages
for Ubuntu Xenial in INSTALL.md

Fixes:#1440

iovisor/bcc,brendangregg/bcc,brendangregg/bcc,brendangregg/bcc,brendangregg/bcc,mcaleavya/bcc,mcaleavya/bcc,mcaleavya/bcc,iovisor/bcc,tuxology/bcc,mcaleavya/bcc,iovisor/bcc,tuxology/bcc,iovisor/bcc,brendangregg/bcc,tuxology/bcc,tuxology/bcc,iovisor/bcc,tuxology/bcc,mcaleavya/bcc