Added Node 15.

.github/workflows/tests.yml

@@ -17,7 +17,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest, windows-latest, macOS-latest]
-        node-version: [10, 12, 14]
+        node-version: [10, 12, 14, 15]
 
     steps:
     - uses: actions/checkout@v2