Handle SECCOMP_OBSELETE event for Ubuntu 12.04 compat (#45)

Firefox 24 hits this event during recording and aborts.  With this patch recording works again.

@rocallahan or @nimrodpar, would either of you care to take a look at this?  This is an area of the code I still feel a bit uncomfortable with.  In particular, I don't know how to write a test for this behavior.  I'm surprised the existing thread test doesn't hit it.