travis.yml: add Go 1.13

.travis.yml

@@ -6,8 +6,5 @@ go:
   - 1.13.x
   - tip
 
-env:
-  - GO111MODULE=off
-
 script:
   - make check

README.md

@@ -45,7 +45,7 @@ default:
 
 With the upcoming [Go2 error proposals](https://go.googlesource.com/proposal/+/master/design/go2draft.md) this package is moving into maintenance mode. The roadmap for a 1.0 release is as follows:
 
-- 0.9. Remove pre Go 1.9 support, address outstanding pull requests (if possible)
+- 0.9. Remove pre Go 1.9 and Go 1.10 support, address outstanding pull requests (if possible)
 - 1.0. Final release.
 
 ## Contributing