Remove .NET Core SDK minor version in .travis.yml

For apt package dotnet-sdk-2.1 - Microsoft .NET Core SDK 2.1.813
diff --git a/.travis.yml b/.travis.yml
index 231c1b4..7027cec 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,7 +1,7 @@
 language: csharp
 solution: apache-nms-amqp.sln
 mono: none
-dotnet: 2.1.202
+dotnet: 2.1
 script:
  - dotnet build -p:AppTargetFramework=netcoreapp2.1 -c Release
  - dotnet test ./test/Apache-NMS-AMQP-Test/Apache-NMS-AMQP-Test.csproj -f netcoreapp2.1 -c Release --filter Category!=Windows
\ No newline at end of file