README: Update Ubuntu required version to 16.04

There is no runc package available on Ubuntu 14.04.

README

@@ -5,7 +5,7 @@ Installing build dependencies
 -----------------------------
 
 To build Tor Browser, you need a Linux distribution that has support
-for runc (such as Debian jessie, Ubuntu 14.04, Fedora 20, etc ...).
+for runc (such as Debian jessie, Ubuntu 16.04, Fedora 20, etc ...).
 On Debian jessie, the runc package is available in backports. On Debian
 stretch, the runc package is available in the main repository.
 Version 0.1.1 of runc is required (which is the version available in