update supported kernel version

Some of the kernel APIs that are used by the core kernel module where
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>
remove mentry mention in kernel version requirement

There are more reasons besides mentry support for 3.9 being the
minimum supported kernel version.  There are also API changes that
happened in 3.9 that make the core module incompatible with previous
versions.

Rather than spelling all that out, just simply state the minimum
version.

Signed-off-by: Seth Jennings <sjenning@redhat.com>
Merge pull request #311 from spartacus06/version-compat-update

update supported kernel version