Add app.config to require .NET 4.6 when launching OmniSharp.exe

[DustinCampbell]

This will cause a dialog to be displayed when launching OmniSharp.exe to inform the user that they need to install .NET Framework 4.6 to run it.

[DustinCampbell]

Note: Without this. starting the net46 version of OmniSharp will result in the following error on machine without .NET 4.6:

Unhandled Exception: System.TypeInitializationException: The type initializer for 'System.Diagnostics.DiagnosticSourceEventSource' threw an exception. ---> System.MissingMethodException: Method not found: 'Void System.Diagnostics.Tracing.EventSource..ctor(System.Diagnostics.Tracing.EventSourceSettings)'.
at System.Diagnostics.DiagnosticSourceEventSource..ctor()
at System.Diagnostics.DiagnosticSourceEventSource..cctor()
--- End of inner exception stack trace ---
at System.Diagnostics.DiagnosticListener..ctor(String name)
at Microsoft.AspNetCore.Hosting.WebHostBuilder.BuildHostingServices()
at Microsoft.AspNetCore.Hosting.WebHostBuilder.Build()
at OmniSharp.Program.Main(String[] args) in C:\Projects\omnisharp-roslyn\src\OmniSharp.Host\Program.cs:line 143
at Program.Main(String[] args) in C:\Projects\omnisharp-roslyn\src\OmniSharp\Program.cs:line 21

This happens on Windows 8.1 with no updates.