Prevent raising an error when `HOMEBREW_RUBY_MACHO=1` is set. Skipping
symlinks and directories while iterating over a directory structure to
find Mach-O binaries makes sense and similar logic is applied elsewhere.
Pulls 'brew linkage' in to main brew repo as a dev-cmd, and has test-bot
use it to detect dylib breakage, which usually means a revision bump is
needed. Checks all dependents, not just those with a 'test do' block
defined, since we can do this without formula support.
Closes#107.
Signed-off-by: Andrew Janke <andrew@apjanke.net>
