security: update :expire time the pub. key security@

The PGP public key is security@theslinux.org. Its old expiration
date is +10 years. Bow it is +2 years (2014-dec-06). This is to
force users to refresh their keys once every 2 years. I do not
what the best expiration date is. 1 year may be too short.

TODO:

1. What is the best expiration date for a PGP public key?
2. Set a cronjob to refresh keys once a day
3. Set a reminder to extend the expiration date for the key
4. Build web of trust, signing party or something like that