Update gcc version requirement

Our README.md stated that gcc 4.7 is required (but 4.8 is recommended), but
4.7 will no longer work now that sched.hh uses, in class wait_object,
constructor inheritence. That feature was only added in 4.8 (see
http://gcc.gnu.org/gcc-4.8/changes.html).
So this patch fixes the README.md to say that gcc 4.8 is required.

Gcc 4.8 is almost a year old, no reason why we shouldn't use it (and the
C++11 features that were still missing in 4.7).

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>