A smarter form of 559cea7fa9.
Travis users can't force Travis to update 10.11 to Xcode 8, so this was murdering
builds left, right & centre.
Fixes https://github.com/Homebrew/brew/issues/1096 whilst still retaining the
point of the original commit. Also offers developers an opt-out so if we need
to test something on 10.11 with Xcode 7.x we can, etc.