ci: test on node 13 (#384)

Author: fisker

.github/workflows/continuous-integration.yml

@@ -38,13 +38,9 @@ jobs:
           - macos-latest
           - windows-latest
         NODE:
+          - 13.x
           - 12.x
           - 10.x
-        exclude:
-          - PLATFORM: macos-latest
-            NODE: 12.x
-          - PLATFORM: windows-latest
-            NODE: 12.x
 
     name: Test on node ${{ matrix.NODE }} and ${{ matrix.PLATFORM }}