The 1.1.5 was a temporary mac only release
The 1.1.6 does not support XP nor vista anymore.
Since the 1.1.3, there was regressions in 1.1.5, and fixes in 1.1.6 of these regressions
And improvements of performances, and compatibility with recent hosts (which are ot supported on XP), and the new midi configs (adds per channel MIDI learn and relative mode).
For the MIDI if you want to use advanced learning feature with 1.1.3, prefer using your DAW.
For the fixes, don't worry 1.1.3 is a good version.
For the better compat with recent host you are not concerned (you don't have cubase 7 nor DP8 on xp).
Tis is the end of an era.
BTW, The support of 10.5 has been dropped too in 1.1.6.
And Apple already announced there drop of 10.6 compat with 10.9.
We currently support this OS, but we don't know if it will be easy to maintain it (it mainly depends on XCode, other DAW compats)...