Update SpiroNet.ViewModels.csproj

[skip ci]

src/SpiroNet.ViewModels/SpiroNet.ViewModels.csproj

@@ -1,6 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFrameworks>netstandard1.1;net45</TargetFrameworks>
+    <TargetFrameworks>netstandard1.3;net45</TargetFrameworks>
     <OutputType>Library</OutputType>
     <EnableDefaultCompileItems>False</EnableDefaultCompileItems>
     <GenerateAssemblyInfo>False</GenerateAssemblyInfo>
@@ -29,6 +29,9 @@
     </Compile>
     <Compile Include="**\*.cs" Exclude="obj\**" />
   </ItemGroup>
+  <ItemGroup>
+    <PackageReference Condition="'$(TargetFramework)' == 'netstandard1.3'" Include="System.IO.FileSystem" Version="4.3.0" />
+  </ItemGroup>
   <ItemGroup>
     <ProjectReference Include="..\SpiroNet.Editor\SpiroNet.Editor.csproj" />
     <ProjectReference Include="..\SpiroNet.Json\SpiroNet.Json.csproj" />