update nuget.exe so package restore works

I wasn't able to build after intial clone. Package restore is failing with the following:

The 'Microsoft.Bcl 1.1.8' package requires NuGet client version '2.8.1' or above, but the current NuGet version is '2.7.41115.310'. The 'Microsoft.Bcl.Async 1.0.168' package requires NuGet client version '2.8.1' or above, but the current NuGet version is '2.7.41115.310'. The 'Microsoft.Bcl.Build 1.0.14' package requires NuGet client version '2.8.1' or above, but the current NuGet version is '2.7.41115.310'.