Change .net core sdk to 2.1 in .travis.yml

Author: cnblogs-dudu

.travis.yml

@@ -3,10 +3,11 @@ mono: none
 sudo: required
 dist: trusty
 before_install:
-  - sudo sh -c 'echo "deb [arch=amd64] https://apt-mo.trafficmanager.net/repos/dotnet-release/ trusty main" > /etc/apt/sources.list.d/dotnetdev.list'
-  - sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 417A0893
+  - 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-2.0.0
+  - sudo apt-get install dotnet-sdk-2.1
 services:
   - memcached  
 addons: