Increase netstandard support to v1.3

src/SixLabors.Shapes/SixLabors.Shapes.csproj

@@ -9,7 +9,7 @@
         <VersionPrefix Condition="$(packageversion) != ''">$(packageversion)</VersionPrefix>
         <VersionPrefix Condition="$(packageversion) == ''">0.1.0-alpha1</VersionPrefix>
         <Authors>Six Labors and contributors</Authors>
-        <TargetFrameworks>netstandard1.1;netstandard2.0</TargetFrameworks>
+        <TargetFrameworks>netstandard1.3;netstandard2.0</TargetFrameworks>
         <LangVersion>7.3</LangVersion>
         <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
         <GenerateDocumentationFile>true</GenerateDocumentationFile>