Enhancement: Vendor dependencies with Go 1.11 Modules

Until now, we've used `dep` for managing dependencies, we've now switch to
using Go modules. For users this does not change much, only if you want to
compile restic without downloading anything with Go 1.11, then you need to run:
`go build -mod=vendor build.go`

https://github.com/restic/restic/pull/1920