README: explicitly state that the instructions are for Ubuntu 14.04 LTS
README: explicitly state that the instructions are for Ubuntu 14.04 LTS

We have always implicitly supported Ubuntu 14.04 LTS as the minimum version.
State it explicitly in the README.

bjackman/trappy,joelagnel/trappy,ARM-software/trappy,joelagnel/trappy,ARM-software/trappy,sinkap/trappy,ARM-software/trappy,sinkap/trappy,bjackman/trappy,sinkap/trappy,sinkap/trappy,bjackman/trappy,bjackman/trappy,ARM-software/trappy,joelagnel/trappy,joelagnel/trappy