shmmax not writing to file and tied it to memeory

Incorrectly using sysctl -p to write setting to file that was
fixed to append the output of the sysctl -w command into
/etc/sysctl.conf.

Tied the shmmax value to 25% of the total RAM in the instance.
Fixes the problem of previously hardcoding it.

Fixes #007
Merge pull request #14 from letterj/tieshmmax2mem

shmmax not writing to file and tied it to memeory