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>

README.md

@@ -42,9 +42,8 @@ dependencies for ``openjdk-7-jdk``
 
 [Oracle JDK]: https://launchpad.net/~webupd8team/+archive/java
 
-To ensure functional C++11 support, Gcc 4.7 or above is required.  Gcc 4.8 or
-above is recommended, as this was the first version to fully comply with the
-C++11 standard.
+To ensure functional C++11 support, Gcc 4.8 or above is required, as this was
+the first version to fully comply with the C++11 standard.
 
 Make sure all git submodules are up-to-date: