update travis to build with 1.14

Signed-off-by: Ivan Porto Carrero <ivan@flanders.co.nz>

.travis.yml

 after_success:
 - bash <(curl -s https://codecov.io/bash)
 go:
-- 1.11.x
-- 1.12.x
+- 1.13.x
+- 1.14.x
 install:
 - GO111MODULE=off go get -u gotest.tools/gotestsum
-env:
-- GO111MODULE=on
 language: go
 notifications:
   slack: