Add Node.js 14 to the CI settings of GitHub Actions (#1253)

Author: abetomo

.github/workflows/tests.yml

@@ -9,7 +9,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        node-version: [8.x, 10.x, 12.x]
+        node-version: [8.x, 10.x, 12.x, 14.x]
         os: [ubuntu-latest, windows-latest, macos-latest]
 
     steps: