Add a separate CI job for testing GMT master branch

Author: seisman

.azure-pipelines.yml

@@ -89,6 +89,10 @@ jobs:
       Python36:
         python.version: '3.6'
         PYTHON: '3.6'
+      GMTMaster:
+        python.version: '3.8'
+        PYTHON: '3.8'
+        CONDA_INSTALL_EXTRA: "codecov"
 
   steps:
 
@@ -116,6 +120,14 @@ jobs:
       conda list
     displayName: List installed packages
 
+  # Install GMT master branch
+  - bash: |
+      set -x -e
+      brew install gmt --HEAD
+      brew install ghostscript
+    displayName: Install GMT master branch
+    condition: eq(variables['System.JobDisplayName'], 'Mac GMTMaster')
+
   # Cache the ${HOME}/.gmt directory, for docs and testing
   - task: Cache@2
     inputs: