Migrate to go mod

[dpordomingo]

caused by https://github.com/src-d/backlog/issues/1392

Since Go v1.12 is already there and both Go v1.11 and Go v1.12 supports Go Modules, we could migrate from dep to go mod

related conversation:

we may have things to discuss (what about vendor?)
from #583 (comment)

There is also a question about go mod tidy
from #583 (comment)