Switch to PackageLicenseExpression in nuget

Author: roji

src/EntityFramework6.Npgsql/EntityFramework6.Npgsql.csproj

@@ -18,7 +18,7 @@
     <PublicSign Condition=" '$(OS)' != 'Windows_NT' ">true</PublicSign>
     <PackageProjectUrl>http://www.npgsql.org</PackageProjectUrl>
     <PackageIconUrl>http://www.npgsql.org/img/postgresql.gif</PackageIconUrl>
-    <PackageLicenseUrl>https://raw.githubusercontent.com/npgsql/npgsql/master/LICENSE.txt</PackageLicenseUrl>
+    <PackageLicenseExpression>PostgreSQL</PackageLicenseExpression>
     <RepositoryType>git</RepositoryType>
     <RepositoryUrl>git://github.com/npgsql/EntityFramework6.Npgsql</RepositoryUrl>
     <Deterministic>true</Deterministic>