Version 4.0 - increment AWS SDK references to 3.5 and support 'dotnet build'

[ashovlin]

Issue #, if available: #19

Description of changes:

• To support building with dotnet msbuild instead of just msbuild:
  • Override FrameworkPathOverride in the net35 csproj per Cannot find reference assemblies for .NET 3.5 or lower using core msbuild dotnet/msbuild#1333
  • Add a dependency on Microsoft.NETFramework.ReferenceAssemblies because the above override path doesn't include System.Web
• Update all NuGet references to latest, including v3.5 AWS SDK references.
• Update version of this library to 4.0 (note - I was unsure whether we wanted to keep matching the SDK with 3.5 or go to 4.0. Happy to adjust.)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.