Update to use go modules (#671)

This includes updates to CI config, vendor files, etc.
I've hard-coded the k8s depedency at 1.13.4 to keep it inline with what
we currently have and to make sure a another run of `go mod tidy`
doesn't accidentally update it to an unexpected version.

Thanks to hectorj2f for carrying this along.

vendor/github.com/json-iterator/go/.travis.yml

@@ -0,0 +1,14 @@
+language: go
+
+go:
+  - 1.8.x
+  - 1.x
+
+before_install:
+  - go get -t -v ./...
+
+script:
+  - ./test.sh
+
+after_success:
+  - bash <(curl -s https://codecov.io/bash)