ext/libcdb/ruby_cdb_writer.c: Remove usage of RARRAY_PTR.

<https://bugs.ruby-lang.org/issues/8399>
ext/libcdb/ruby_cdb_reader.[ch]: Remove usage of RARRAY_PTR.

<https://bugs.ruby-lang.org/issues/8399>

NOTE: Dropped support for Ruby 1.8.7 in the process due to switching to rb_block_call instead of rb_iterate (which is obsolete since 1.9).
lib/libcdb.rb (CDB.open): Don't clobber original error on close.
Updated ChangeLog.