Use latest az tools installation methodology

Updates to the latest directions from https://docs.microsoft.com/en-us/cli/azure/install-azure-cli-apt?view=azure-cli-latest#signingKey

.travis.yml

@@ -52,8 +52,10 @@ install:
 - export PATH="$PWD/.dotnet:$PATH"
 after_success: |
   if [[ "$TRAVIS_OS_NAME" == "linux" && $TRAVIS_BRANCH == "master" && $TRAVIS_PULL_REQUEST == "false" ]]; then
-    echo "deb [arch=amd64] https://packages.microsoft.com/repos/azure-cli/ wheezy main" | sudo tee /etc/apt/sources.list.d/azure-cli.list
-    sudo apt-key adv --keyserver packages.microsoft.com --recv-keys 52E16F86FEE04B979B07E28DB02C46DF417A0893
+    AZ_REPO=$(lsb_release -cs)
+    echo "deb [arch=amd64] https://packages.microsoft.com/repos/azure-cli/ $AZ_REPO main" | \
+      sudo tee /etc/apt/sources.list.d/azure-cli.list
+    curl -L https://packages.microsoft.com/keys/microsoft.asc | sudo apt-key add -
     sudo apt-get install apt-transport-https
     sudo apt-get update && sudo apt-get install azure-cli