Update NLog version to 2.1.8-beta

Author: Todd Lair

Src/NLog.Targets.Stackify/NLog.Targets.Stackify.csproj

@@ -15,7 +15,7 @@
     <GenerateAssemblyCopyrightAttribute>false</GenerateAssemblyCopyrightAttribute>
     <GenerateAssemblyVersionAttribute>false</GenerateAssemblyVersionAttribute>
     <GenerateAssemblyFileVersionAttribute>false</GenerateAssemblyFileVersionAttribute>
-    <Version>2.1.7</Version>
+    <Version>2.1.8-beta</Version>
     <PackageLicenseUrl>https://github.com/stackify/stackify-api-dotnet/blob/master/LICENSE</PackageLicenseUrl>
     <PackageProjectUrl>https://github.com/stackify/stackify-api-dotnet</PackageProjectUrl>
     <PackageIconUrl>https://stackify.com/wp-content/uploads/2017/02/stk.png</PackageIconUrl>