[nunit] Set nuget version to use as 2.6.4
[nunit] Set nuget version to use as 2.6.4

AriSuutariST/waffle,Waffle/waffle,Waffle/waffle,AriSuutariST/waffle,hazendaz/waffle,AriSuutariST/waffle,hazendaz/waffle,hazendaz/waffle,Waffle/waffle,dblock/waffle,dblock/waffle,dblock/waffle,dblock/waffle,AriSuutariST/waffle