improve ci go version(go version in go.mod not change)

.github/workflows/github-actions.yml

@@ -19,7 +19,7 @@ jobs:
         os:
           - ubuntu-latest
         go_version:
-          - 1.13
+          - 1.15
     env:
       DING_TOKEN: ${{ secrets.DING_TOKEN }}
       DING_SIGN: ${{ secrets.DING_SIGN }}

.travis.yml

 language: go
 
 go:
-  - "1.13.x"
+  - "1.15.x"
 
 script:
   - go fmt ./... && [[ -z `git status -s` ]]