use go1.11 with go mod

vendor/github.com/gin-gonic/gin/.travis.yml

@@ -1,22 +0,0 @@
-language: go
-sudo: false
-go:
-  - 1.4
-  - 1.5.4
-  - 1.6.4
-  - 1.7.4
-  - tip
-
-script:
-  - go test -v -covermode=count -coverprofile=coverage.out
-
-after_success:
-  - bash <(curl -s https://codecov.io/bash)
-
-notifications:
-  webhooks:
-    urls:
-      - https://webhooks.gitter.im/e/7f95bf605c4d356372f4
-    on_success: change  # options: [always|never|change] default: always
-    on_failure: always  # options: [always|never|change] default: always
-    on_start: false     # default: false