When building docs, we compute the release branch for the product, based

on the version. The code we had for this was using indexes into

reviewboard.VERSION, and had a wrong index used in one spot.
This was hit when building beta docs, as it's a code path we don't usually

hit during doc building.
This change bullet-proofs this logic by pulling out all the variants

from VERSION up-front and then uses those for the logic, instead of

index lookups.

Successfully built the docs.