Bump go to 1.15 in CI

Author: jmrodri

.github/workflows/ci.yml

@@ -16,7 +16,7 @@ jobs:
     - name: Set up Go 1.x
       uses: actions/setup-go@v2
       with:
-        go-version: ^1.13
+        go-version: ^1.15
       id: go
 
     - name: Check out code into the Go module directory