Fixed the .NET 4.0 build.
Fixed the .NET 4.0 build.

CaioProiete/serilog,serilog/serilog,richiej84/serilog,clarkis117/serilog,adamchester/serilog,DavidSSL/serilog,redwards510/serilog,ajayanandgit/serilog,serilog/serilog,nblumhardt/serilog,colin-young/serilog,joelweiss/serilog,joelweiss/serilog,clarkis117/serilog,merbla/serilog,harishjan/serilog,ravensorb/serilog,skomis-mm/serilog,nblumhardt/serilog,Applicita/serilog,colin-young/serilog,vossad01/serilog,SaltyDH/serilog,ravensorb/serilog,harishjan/serilog,merbla/serilog,adamchester/serilog,JuanjoFuchs/serilog,skomis-mm/serilog,zmaruo/serilog,Jaben/serilog,khellang/serilog,vorou/serilog,jotautomation/serilog,richardlawley/serilog