ruby: Support 'make INSTALLDIRS=vendor install' for Ruby

(This is the same change as 87c9ec881cb695724e01d9f6fc9df996d4c67cb6
in libguestfs.)
ruby: Fix fallback to rake/rdoctask for old rake.

(This is the same change as 480bd3a6d8a56b37936b08c94eb99f0921fe65de
in libguestfs.)