chore: remove go1.8/1.9 and support go1.13

.travis.yml

@@ -1,11 +1,10 @@
 language: go
 sudo: false
 go:
-  - 1.8.x
-  - 1.9.x
   - 1.10.x
   - 1.11.x
   - 1.12.x
+  - 1.13.x
   - master
 
 git: