ci: add Go 1.15.x

Author: unknwon

.github/workflows/go.yml

@@ -21,7 +21,7 @@ jobs:
     name: Test
     strategy:
       matrix:
-        go-version: [1.13.x, 1.14.x]
+        go-version: [1.13.x, 1.14.x, 1.15.x]
         platform: [ubuntu-latest, macos-latest, windows-latest]
     runs-on: ${{ matrix.platform }}
     steps: