Modify .travis.yml for .net core 2.2

Author: cnblogs-dudu

.travis.yml

@@ -1,13 +1,8 @@
 language: csharp
 mono: none
 sudo: required
-dist: trusty
-before_install:
-  - curl -O https://packages.microsoft.com/config/ubuntu/14.04/packages-microsoft-prod.deb
-  - sudo dpkg -i packages-microsoft-prod.deb
-  - sudo apt-get install apt-transport-https
-  - sudo apt-get update
-  - sudo apt-get install dotnet-sdk-2.1
+dist: xenial
+dotnet: 2.2
 services:
   - memcached  
 addons:
@@ -19,11 +14,6 @@ env:
     - DOTNET_CLI_TELEMETRY_OPTOUT: 1
 os:
   - linux
-branches:
-  only:
-    - master
-    - dotnetcore
-    - dotnetcore2-preview2
 script:
   - ./build.sh
   - ./test.sh