ruby: Pass --without-gmp unless build.with? "gmp"

Ruby will pick up gmp if it's on your machine, unless it's
specifically told to not look for gmp.  This is true regardless of
whether the user invokes the Homebrew flag --with-gmp.  It's safest to
force ruby to not look for gmp unless the user has specifically
requested gmp with the Homebrew option.  Otherwise, if the user 
happens to brew ruby with gmp already on the machine, then later
removes gmp, ruby will bomb out with a "dylib not found" error.