Set hard version for Serilog Dependency

Author: merbla

src/Serilog.Sinks.Splunk/Serilog.Sinks.Splunk.nuspec

@@ -12,7 +12,7 @@
     <tags>serilog logging splunk</tags>
     <dependencies>
       <group>
-        <dependency id="Serilog" version="$version$" />
+        <dependency id="Serilog" version="1.4.168" />
         <dependency id="Splunk.Client" version="2.0.0" />
       </group>
       <group targetFramework="net45">