Use composition to be compatible with Ruby 1.9.3

```
[1] pry(main)> RUBY_VERSION
=> "1.9.3"
[2] pry(main)> require 'bigdecimal'
=> true
[3] pry(main)> class A < BigDecimal
[3] pry(main)*   def initialize *args
[3] pry(main)*     puts "in A"
[3] pry(main)*     super
[3] pry(main)*   end
[3] pry(main)* end
=> nil
[4] pry(main)> a = A.new 0
=> #<BigDecimal:4140dd0,'0.0',9(36)>
[5] pry(main)> a.class
=> BigDecimal
[6] pry(main)> a.is_a? A
=> false
```