Add ``MAGIC_SYSRQ`` to ``contrib/check-config.sh``. Required kernel conf...

...ig for Docker 1.x

Docker-DCO-1.1-Signed-off-by: James Mills <prologic@shortcircuit.net.au>
(github: therealprologic)
Considering the fact that above that line, tabs are used, could you please use tabs for indentation here?