Pull in latest PJSIP.

This includes two fixes:

* https://github.com/Metaswitch/pjsip-upstream/pull/2 Correct handling of exceptional events

* https://github.com/Metaswitch/pjsip-upstream/pull/4 Set endpoint pool chunk size to 10MB
Merge pull request #57 from Metaswitch/get-latest-pjsip

[No review required] Pull in latest PJSIP.