CI: Replace image macOS-10.14 with macOS-10.13

macOS-10.14 fails when importing simplejson & markupsafe built
extensions.

Author: jayvdb

ci/azure-pipelines.yml

@@ -7,7 +7,7 @@ jobs:
 - template: azure-pipelines-template.yml
   parameters:
     name: macOS
-    vmImage: macOS-10.14
+    vmImage: macOS-10.13
 
 - template: azure-pipelines-template.yml
   parameters: