update supported kernel version

Some of the kernel APIs that are used by the core kernel module were
updated in 3.9 and are incompatible with previous kernel versions.

Update the README to reflect this.

Fixes #257

Signed-off-by: Seth Jennings <sjenning@redhat.com>
Should we remove mention of -mfentry?  IIRC there are really at least 3 reasons why we need these versions of gcc and kernel.