Added support for .NET Standard 1.1 and 1.2

src/Discord.Net.Rpc/DiscordRpcConfig.cs

@@ -1,5 +1,6 @@
 using Discord.Net.WebSockets;
 using Discord.Rest;
+using System;
 
 namespace Discord.Rpc
 {
@@ -14,6 +15,19 @@ namespace Discord.Rpc
         public int ConnectionTimeout { get; set; } = 30000;
 
         /// <summary> Gets or sets the provider used to generate new websocket connections. </summary>
-        public WebSocketProvider WebSocketProvider { get; set; } = () => new DefaultWebSocketClient();
+        public WebSocketProvider WebSocketProvider { get; set; }
+
+        public DiscordRpcConfig()
+        {
+#if NETSTANDARD1_3
+            WebSocketProvider = () => new DefaultWebSocketClient();
+#else
+            WebSocketProvider = () =>
+            {
+                throw new InvalidOperationException("The default websocket provider is not supported on this platform.\n" +
+                    "You must specify a WebSocketProvider or target a runtime supporting .NET Standard 1.3, such as .NET Framework 4.6+.");
+            };
+#endif
+        }
     }
 }