fix(auth): only accept v3 X.509 certificates

v1 and v2 are forbidden as at least one extension is required.  Instead
of actually parsing the version number, just compare it with a
hard-coded string.

Change-Id: Ib8fd34304a0049787db77ec8c2359d0930cd4ba1
Signed-off-by: Demi Marie Obenour <demiobenour@gmail.com>