Increase macOs CI coverage to include both 10.14 and 10.15 as 10.15 has recently been added to Azure DevOps hosted agents (#1412)

Author: bergmeister

.azure-pipelines-ci/ci.yaml

@@ -29,8 +29,10 @@ stages:
               vmImage: ubuntu-16.04
             Ubuntu_18_04:
               vmImage: ubuntu-18.04
-            macOS:
-              vmImage: macos-latest
+            macOS_10_14_Mojave:
+              vmImage: macOS-10.14
+            macOS_10_15_Catalina:
+              vmImage: macOS-10.15
             Windows_Server2016_PowerShell_Core:
               vmImage: vs2017-win2016
             Windows_Server2019_PowerShell_Core: