Make rbintegrations a packaging-only dependency.

Review Request #907 — Created Oct. 13, 2017 and submitted

Review Board
abcd, all
In 3.0 beta 1, we attempted to make rbintegrations a dependency of
Review Board, which worked fine for the packaging process but created an
issue when building an environment to package rbintegrations. Since
rbintegrations was a hard dependency of Review Board, it created a
circular reference issue.

Now, we only include rbintegrations as a dependency if actively building
a package (through a `sdist` or `bdist` command), or if installing
(using `install`). Setting up a development environment no longer
requires it.

Testing Done:
Tested running `develop` and saw that `rbintegrations` wasn't a

Tested the packaging commands and inspected the package metadata.
`rbintegrations` was a dependency.

Reviewed at

Review request changed

Status: Closed (submitted)