Update mac Image to 10.14 in CI as 10.13 is going to be deprecated in March (#1343)

Author: bergmeister

az-ci/azure-pipelines.yml

@@ -8,7 +8,7 @@ jobs:
 - template: azure-devops-psreadline.yml
   parameters:
     name: macOS
-    vmImage: 'macOS-10.13'
+    vmImage: 'macOS-10.14'
 
 - template: azure-devops-psreadline.yml
   parameters: