Added required dependency to nuspec for net45 (System.Collections.Immutable) (#109)

Author: blurt

grunt/templates/SocketIoClientDotNet.nuspec

@@ -26,6 +26,7 @@
         <dependency id="SuperSocket.ClientEngine.Core" version="0.8.0.12" />
         <dependency id="Newtonsoft.Json" version="9.0.1" />
         <dependency id="EngineIoClientDotNet" version="1.0.0.1-beta1" />
+        <dependency id="System.Collections.Immutable" version="1.3.1" />
       </group>
       <group targetFramework="netstandard1.3">
         <dependency id="WebSocket4Net" version="0.15.0-beta9" />