Sure,
2) Add the configuration you provided to Raven.Server.exe.config updating the Npgsql version
<system.data>
<DbProviderFactories>
<add name="Npgsql Data Provider" invariant="Npgsql" support="FF" description=".Net Framework Data Provider for Postgresql Server" type="Npgsql.NpgsqlFactory, Npgsql, Version=2.1.3.0, Culture=neutral, PublicKeyToken=5d8b90d52f46fda7" />
</DbProviderFactories>
</system.data>
3) Restart the server and... it works!
I didn't have to add the DLLs to the GAC which is great.