Determine minimal gcc version for echo_chamber and modify cmake to enforce it.

[inetic]

It seems g++ 4.8 doesn't support some random features that the echo_chamber is using.

/home/ubuntu/np1sec/test/echo_chamber/tests.cc:547:10: error: ‘random_device’ in namespace ‘std’ does not name a type
     std::random_device rd;
 
(16:28:01) donncha: 
/home/ubuntu/np1sec/test/echo_chamber/tests.cc:548:10: error: ‘mt19937’ in namespace ‘std’ does not name a type
     std::mt19937 gen;