.travis.yml: Bugfix for no module support in CI

Tip of dependency is broken and it was affecting us:

https://github.com/go-errors/errors/issues/27

- We also disabled 1.10 and 1.11 due to lack of support in dependencies now that we're being recognized as a module.

.travis.yml

@@ -4,8 +4,8 @@ go:
   - stable
   - "1.13"
   - "1.12"
-  - "1.11"
-  - "1.10"
+env:
+  - GO111MODULE=on
 install:
   - go get -t ./...
   - go get github.com/mattn/goveralls