fix travis, 1.10 doesnt support by unconvert anymore

.travis.yml

@@ -1,10 +1,13 @@
 language: go
 go_import_path: github.com/pkg/errors
 go:
-  - 1.10.x
   - 1.11.x
   - 1.12.x
+  - 1.13.x
   - tip
 
+env:
+  - GO111MODULE=off
+
 script:
   - make check