Detect when rebuilds are needed

[fredemmott]

Homebrew periodically updates dependencies, and old versions are not installable. HHVM needs rebuilding when this happens.

It would be good to automate a check for this; this could either look at our dependencies (brew deps) and when they were last updated, or try installing the bottle and executing a hello world hack file + executing the typechecker, and triggering rebuilds for all supported releases if necessary