Update changelog

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
OpenConnectManagementThread: Enforce a minimum MTU

Several users reported that they were running into bugs in Android
4.4.3/4.4.4 which caused VpnService.Builder.establish() to fail
catastrophically with low-ish (1200) MTUs.  So:

For IPv4 connections on <= 4.3.x, the new floor is 576.
For IPv6 connections anywhere, or for >= 4.4.x, the new floor is 1280.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>