CI: test on node 15 (#2836)

Author: IvanGoncharov

.github/workflows/ci.yml

@@ -145,7 +145,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        node_version_to_setup: [10, 12, 14]
+        node_version_to_setup: [10, 12, 14, 15]
     steps:
       - name: Checkout repo
         uses: actions/checkout@v2

integrationTests/node/package.json

@@ -7,6 +7,7 @@
     "graphql": "file:../graphql.tgz",
     "node-10": "npm:[email protected]",
     "node-12": "npm:[email protected]",
-    "node-14": "npm:[email protected]"
+    "node-14": "npm:[email protected]",
+    "node-15": "npm:[email protected]"
   }
 }