update nuget.exe so package restore works

I wasn't able to build after a clone. Package restore is failing when
with the following:

NuGet.exe restore MediaBrowser.sln

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'.