ext/libcdb/ruby_cdb_{reader,writer}.c: Eliminate use of RHASH_TBL.

Not supported in Rubinius. We only get string values anyway, so it's safe to test for nil.
ext/libcdb/ruby_cdb_reader.c: Eliminate use of rb_fix2str.

Not supported in Rubinius.
spec/libcdb/reader_spec.rb: Skip failing examples on Rubinius.

All errors seem to boil down to `rb_block_call` via `RCDB_READER_ITERATE*`.