Update dotnet version.

Denis Ivanov · Denis Ivanov · commit e0be84956c19 · 2019-11-16T16:18:43.000+04:00
diff --git a/.travis.yml b/.travis.yml
@@ -1,6 +1,6 @@
 language: csharp
 solution: src/AngleSharp.XPath.sln
-dotnet  : 2.0.3
+dotnet  : 3.0.100
 script  :
   - dotnet restore src/AngleSharp.XPath.sln
   - dotnet build src/AngleSharp.XPath.sln
