Upgrade System.Data.SqlClient to 4.4.0-* (#300)

- Addresses "SqlClient fails with netcoreapp2.0 on Win7/Server2008" (https://github.com/dotnet/corefx/issues/18406)
- Revert if and when $(CoreFxVersion) is upgraded to 4.4.0-*

Author: mikeharder

src/Microsoft.Extensions.Caching.SqlServer/Microsoft.Extensions.Caching.SqlServer.csproj

@@ -22,7 +22,12 @@
   </ItemGroup>
 
   <ItemGroup Condition=" '$(TargetFramework)' == 'netstandard1.2' ">
-    <PackageReference Include="System.Data.SqlClient" Version="$(CoreFxVersion)" />
+    <!--
+      * Use 4.4.0-* instead of $(CoreFxVersion) to address "SqlClient fails with netcoreapp2.0 on Win7/Server2008"
+        * https://github.com/dotnet/corefx/issues/18406
+      * Revert if and when $(CoreFxVersion) is upgraded to 4.4.0-*
+    -->
+    <PackageReference Include="System.Data.SqlClient" Version="4.4.0-*" />
   </ItemGroup>
 
 </Project>