I've tried to switch runtime to .net 4.6 and receive errors in PlayFab editor extension:
https://gist.github.com/merlineus/20fab25f1d6d642dabe87c8a55a96880
https://gist.github.com/merlineus/7f2d07d555cc543f8ad4b18f1f7053c4
https://gist.github.com/merlineus/b021ec06a5e3f248d3f0309208deadcd
And PlayFab SDK:
https://gist.github.com/merlineus/32a0dd330b8260b09851fbd0901fbbbf
.net 4.6 (and maybe .netstandard 2.0) is going to be stable in Unity 2017.2 so it's a good idea to adopt your code for them.