Fix random_compat on php 7.0 (#111)

random_compat on php7 can only install version 9.99.99 (which is a noop version). Requiring >=2 allows 2.0 to install on php5 while allowing 9.99.99 on php7+. See: https://github.com/paragonie/random_compat#version-conflict-with-other-php-project