Upgrade test project to .NET Core 2.1 (chadly#124)

https://docs.microsoft.com/en-us/dotnet/core/whats-new/dotnet-core-2-1

Author: pranav700

test/Geocoding.Tests/Geocoding.Tests.csproj

@@ -3,11 +3,11 @@
   <PropertyGroup>
     <AssemblyTitle>Geocoding.net Tests</AssemblyTitle>
     <VersionPrefix>4.0.0-beta1</VersionPrefix>
-    <TargetFramework>netcoreapp1.1</TargetFramework>
+    <TargetFramework>netcoreapp2.1</TargetFramework>
     <AssemblyName>Geocoding.Tests</AssemblyName>
     <PackageId>Geocoding.Tests</PackageId>
     <GenerateRuntimeConfigurationFiles>true</GenerateRuntimeConfigurationFiles>
-    <RuntimeFrameworkVersion>1.1.1</RuntimeFrameworkVersion>
+    <RuntimeFrameworkVersion>2.1.0</RuntimeFrameworkVersion>
     <Version>4.0.0</Version>
     <Description>Tests</Description>
   </PropertyGroup>