Add missing github.com/gorilla/handlers dep

vendor/github.com/gorilla/handlers/LICENSE

vendor/github.com/gorilla/handlers/README.md

vendor/github.com/gorilla/handlers/canonical.go

vendor/github.com/gorilla/handlers/compress.go

vendor/github.com/gorilla/handlers/cors.go

vendor/github.com/gorilla/handlers/doc.go

vendor/github.com/gorilla/handlers/handlers.go

vendor/github.com/gorilla/handlers/handlers_go18.go

vendor/github.com/gorilla/handlers/handlers_pre18.go

vendor/github.com/gorilla/handlers/proxy_headers.go

vendor/github.com/gorilla/handlers/recovery.go

vendor/vendor.json