Build against .net core 1.1 for current CI

tests/SkiaSharp.NetCore.Tests/SkiaSharp.NetCore.Tests.csproj

@@ -1,7 +1,7 @@
 <?xml version="1.0" encoding="utf-8"?>
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFramework>netcoreapp2.0</TargetFramework>
+    <TargetFramework>netcoreapp1.1</TargetFramework>
     <DefineConstants>$(DefineConstants);NET_STANDARD</DefineConstants>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
     <DebugType>portable</DebugType>
@@ -9,7 +9,7 @@
     <PackageId>SkiaSharp.NetCore.Tests</PackageId>
     <GenerateRuntimeConfigurationFiles>true</GenerateRuntimeConfigurationFiles>
     <RuntimeIdentifiers>win7-x64;win7-x86;osx.10.10-x64;osx.10.11-x64;ubuntu.14.04-x64;ubuntu.14.10-x64;ubuntu.16.04-x64;ubuntu.16.10-x64</RuntimeIdentifiers>
-    <RuntimeFrameworkVersion>2.0.0</RuntimeFrameworkVersion>
+    <RuntimeFrameworkVersion>1.1.1</RuntimeFrameworkVersion>
   </PropertyGroup>
   <ItemGroup>
     <PackageReference Include="xunit" Version="2.3.1" />