upgraded to Serilog v2.7.1 (#54)

Author: Aaronontheweb

src/Akka.Logger.Serilog/Akka.Logger.Serilog.csproj

@@ -10,8 +10,8 @@
   </PropertyGroup>
 
   <ItemGroup>
+    <PackageReference Include="Serilog" Version="2.7.1" />
     <PackageReference Include="Akka" Version="1.3.9" />
-    <PackageReference Include="Serilog" Version="2.6.0" />
   </ItemGroup>
 
   <PropertyGroup Condition=" '$(Configuration)' == 'Release' ">