Remove go versions in CI with no strings.Builder

.travis.yml

@@ -1,9 +1,10 @@
 sudo: false
 language: go
 go:
-  - 1.7.x
-  - 1.8.x
-  - 1.9.x
   - 1.10.x
   - 1.11.x
+  - 1.12.x
+  - 1.13.x
+  - 1.14.x
+  - 1.15.x
   - master