housekeeping: use https

Directory.build.props

@@ -1,7 +1,7 @@
 <Project>
   <PropertyGroup>
     <Copyright>Copyright (c) .NET Foundation and Contributors</Copyright>
-    <PackageLicenseUrl>http://opensource.org/licenses/ms-pl</PackageLicenseUrl>
+    <PackageLicenseUrl>https://opensource.org/licenses/ms-pl</PackageLicenseUrl>
     <PackageProjectUrl>https://github.com/MvvmCross</PackageProjectUrl>
     <PackageIconUrl>http://i.imgur.com/Baucn8c.png</PackageIconUrl>
     <Authors>.NET Foundation and Contributors</Authors>