Update travis

.travis.yml

@@ -3,7 +3,7 @@ solution: MQTTnet.sln
 
 matrix:
   include:
-    - dotnet: 1.0.1
+    - dotnet: 2.0.0
       mono: none
       dist: trusty
       env: DOTNETCORE=1  # optional, can be used to take different code paths in your script