chore: continue testing on Node 15 (#360)

Author: nickserv

.github/workflows/validate.yml

@@ -16,7 +16,7 @@ jobs:
     if: ${{ !contains(github.head_ref, 'all-contributors') }}
     strategy:
       matrix:
-        node: [10.14, 12, 14, 16]
+        node: [10.14, 12, 14, 15, 16]
     runs-on: ubuntu-latest
     steps:
       - name: ⬇️ Checkout repo