Use dep to manage dependencies

Review Request #9565 — Created Feb. 1, 2018 and submitted — Latest diff uploaded

Information

rb-gateway
master
0b594f8...

Reviewers

The new dep experiment (pending merger into the go toolchain with go
1.10) allows us to generate a lockfile à la npm or cargo so that we can
pin our dependencies. This also allows us to specify branches for said
dependencies, so we can use the upstream libgit2/git2go repository
pinned at branch v24 instead of relying on the gopkg.in service.

Built rb-gateway with dep.
Ran unit tests.