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/spf13/pflag/.travis.yml

@@ -0,0 +1,21 @@
+sudo: false
+
+language: go
+
+go:
+  - 1.7.3
+  - 1.8.1
+  - tip
+
+matrix:
+  allow_failures:
+    - go: tip
+
+install:
+  - go get github.com/golang/lint/golint
+  - export PATH=$GOPATH/bin:$PATH
+  - go install ./...
+
+script:
+  - verify/all.sh -v
+  - go test ./...