Update WS4Net dependency to 0.15.0 (#812)

Also changes target to NetStandard1.3

src/Discord.Net.Providers.WS4Net/Discord.Net.Providers.WS4Net.csproj

@@ -4,12 +4,12 @@
     <AssemblyName>Discord.Net.Providers.WS4Net</AssemblyName>
     <RootNamespace>Discord.Providers.WS4Net</RootNamespace>
     <Description>An optional WebSocket client provider for Discord.Net using WebSocket4Net</Description>
-    <TargetFramework>net45</TargetFramework>
+    <TargetFramework>netstandard1.3</TargetFramework>
   </PropertyGroup>
   <ItemGroup>
     <ProjectReference Include="..\Discord.Net.Core\Discord.Net.Core.csproj" />
   </ItemGroup>
   <ItemGroup>
-    <PackageReference Include="WebSocket4Net" Version="0.14.1" />
+    <PackageReference Include="WebSocket4Net" Version="0.15.0" />
   </ItemGroup>
-</Project>
+</Project>