Fix missing method exception System.Net.Sockets.Socket..ctor in Mono

Mono has no default constructor for Socket class introduced in .NET 4.5
and KRuntime fails with:
`Missing method System.Net.Sockets.Socket::.ctor(SocketType,ProtocolType)
in assembly
  /usr/local/Cellar/kmono/3.6.1-10002/lib/mono/gac/System/4.0.0.0__b77a5c561934e089/System.dll,
referenced in assembly
  /Users/mattroberts/.kre/packages/KRE-mono45-x86.1.0.0-alpha4-10274/bin/Microsoft.Framework.Runtime.dll
Method not found: 'System.Net.Sockets.Socket..ctor'.`

Fixed with change of constructor call to existing on + setting of
DualMode property

And seems not just me struggling with that! http://stackoverflow.com/questions/25312521/trying-to-get-aspnet-vnext-working-on-a-mac-missing-method-errors