Upgraded my PGP key

For security reasons, upgraded my PGP/GPG key to 4096-bit RSA.

Key ID: 2F39D84D
Key fingerprint: BB13 C3CE A321 09A1 B93C BF49 2365 E976 2F39 D84D

ref: https://lkml.org/lkml/2011/9/30/421