Add automake to Linux Prerequisites

[rzana]

Building libevent requires aclocal, that can be found in automake. A simple

sudo apt-get install automake

will do the job.

[cretz]

I do mention sudo apt-get install libtool which should include the automake package I believe. Can you confirm?

EDIT: I see it's only a suggestion, ok, will add. Travis seems to succeed without it but I assume it's already present on their images.

[rzana]

I've just checked my terminal and I did run sudo apt-get install libtool successfully before (along with installing build-essential and autopoint), so it seems automake has to be installed separately.