Update Serilog dependency to 2.2.0

Author: nblumhardt

src/Serilog.Sinks.File/project.json

@@ -9,7 +9,7 @@
     "iconUrl": "http://serilog.net/images/serilog-sink-nuget.png"
   },
   "dependencies": {
-    "Serilog": "2.2.0-dev-00688"
+    "Serilog": "2.2.0"
   },
   "buildOptions": {
     "keyFile": "../../assets/Serilog.snk",