Brought back PowerShell's AssemblyLoadContext

Removes dependency on Microsoft.Extensions.Platform.Abstractions.

Makes use of native executable from `dotnet-cli` impossible; must use
`powershell` from monad-native.

src/Microsoft.PowerShell.Commands.Utility/project.json

@@ -122,7 +122,6 @@
     "frameworks": {
         "dnxcore50": {
             "dependencies": {
-                "Microsoft.Extensions.PlatformAbstractions": "1.0.0-*",
                 "Microsoft.CSharp": "4.0.1-*",
                 "Microsoft.VisualBasic": "10.0.1-*",
                 "Microsoft.Win32.Primitives": "4.0.1-*",