[SL.Core] Increase minimium netstandard support to v1.3

src/SixLabors.Core/Helpers/HashHelpers.cs

@@ -1,8 +1,6 @@
 // Copyright (c) Six Labors and contributors.
 // Licensed under the Apache License, Version 2.0.
 
-using System;
-
 namespace SixLabors
 {
     /// <summary>

src/SixLabors.Core/SixLabors.Core.csproj

@@ -5,7 +5,7 @@
         <VersionPrefix Condition="$(packageversion) != ''">$(packageversion)</VersionPrefix>
         <VersionPrefix Condition="$(packageversion) == ''">0.1.0-alpha2</VersionPrefix>
         <Authors>Six Labors</Authors>
-        <TargetFrameworks>netstandard1.1;netstandard2.0;netcoreapp2.0;netcoreapp2.1;</TargetFrameworks>
+        <TargetFrameworks>netstandard1.3;netstandard2.0;netcoreapp2.0;netcoreapp2.1;</TargetFrameworks>
         <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
         <GenerateDocumentationFile>true</GenerateDocumentationFile>
         <AssemblyName>SixLabors.Core</AssemblyName>