use go modules for dependencies

.travis.yml

@@ -5,7 +5,5 @@ go:
   - '1.10'
   - '1.11'
 before_install:
-  - go get -v github.com/google/go-querystring/query
-  - go get -v github.com/h2non/gock
-  - go get -v github.com/stretchr/testify/assert
+  - go mod tidy
 script: go test -v ./...