Make rbintegrations a packaging-only dependency.
Review Request #907 - Created Oct. 13, 2017 and submitted
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 dependency. Tested the packaging commands and inspected the package metadata. `rbintegrations` was a dependency. Reviewed at https://reviews.reviewboard.org/r/9249/