Rename gitlab to git-lab

[ordian]

Follow-up on comment.

[imp]

Documentation at http://doc.crates.io/manifest.html#package-metadata still mentions "gitlab" instead of the "git-lab"

[ordian]

Documentation is located in cargo repository. I'm not sure this change is already deployed.

[ordian]

@sgrif I just noticed, that name git-lab is inconsistent with coverage services:
codecov = { repository = "...", branch = "master", service = "gitlab" }
maybe we should revert this change?

[sgrif]

@ordian My intention was for this change to only affect how we store things internally and how it's sent to the front-end, not affect what's allowed in Cargo.toml. I see now that this isn't doing that. I'm going to go ahead and revert. We could probably fix it, but honestly this probably isn't worth the effort for such a small change.