update travis to 5.0.100

.travis.yml

@@ -2,7 +2,7 @@ language: csharp
 sudo: required
 dist: xenial
 solution: CAP.sln
-dotnet: 3.1.100
+dotnet: 5.0.100
 mono: none
 env:
  - Cap_MySql_ConnectionString="Server=127.0.0.1;Database=cap_test;Uid=root;Pwd=;Allow User Variables=True;SslMode=none"