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/google/gofuzz/.travis.yml

@@ -0,0 +1,13 @@
+language: go
+
+go:
+  - 1.4
+  - 1.3
+  - 1.2
+  - tip
+
+install:
+  - if ! go get code.google.com/p/go.tools/cmd/cover; then go get golang.org/x/tools/cmd/cover; fi
+
+script:
+  - go test -cover