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/imdario/mergo/.travis.yml

@@ -0,0 +1,7 @@
+language: go
+install:
+  - go get -t
+  - go get golang.org/x/tools/cmd/cover
+  - go get github.com/mattn/goveralls
+script:
+  - $HOME/gopath/bin/goveralls -service=travis-ci -repotoken $COVERALLS_TOKEN