Be nice in the presence of method aliasing

Flushing each fetched AST from the cache makes it impossible to re-fetch
the ast for a method under a different name. This change disables this
automatic flushing. The effect on memory should be minimal, as even in
the presence of flushing the ASTs still stick around, bound to their
methods.
Update README to match changed flushing behavior
Record changed fetching behavior in the change log
Do not destroy the AST when creating ruby source