ibex-2.8.6

[soonhokong]

We've found some issues with the current version of ibex that we're using (ibex-2.7.4 + custom patches). We will use ibex-2.8.6 instead.

Action items:

• Update libibex-dev in https://launchpad.net/~dreal/+archive/ubuntu/dreal.
• Provide ibex@2.8.6 in https://github.com/RobotLocomotion/homebrew-director/tree/master/Formula. This requires generating bottles.
• There should be small changes in dreal regarding how we use IBEX's polytope contractor.

[soonhokong]

@BetsyMcPhail, I'll ping you when ibex@2.8.6 formula is ready.

[jwnimmer-tri]

@soonhokong Currently Drake is building against ibex-2.8.6_4.

That release is a branch name (not a tag name), which means that the GitHub archive will not have a stable checksum (see bazel-contrib/SIG-rules-authors#11 (comment)), and also means that the branch contents could change out from under us if someone pushes a new commit onto the branch.

I'd like to request that dreal-deps/ibex-lib goes back to using tag names to denote its releases, and in particular that an existing release (either 2.8.6-4 or perhaps 2.8.6-5) be added as a tag name.