replace "dep" with "go mod"

A simple "go mod init && go mod tidy && go mod vendor" was enough and
mostly seems to have picked the same revisions of the dependencies as
before. Notable exceptions are klog and gogo/protobuf.

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)