MSON AST reflecting the new specification

For @fosrias, @honzajavorek and @pksunkara to review. 
> This is plural in the spec. Do we need to change that?

Interesting we do not even have `Validations` in [keywords](https://github.com/apiaryio/mson/blob/master/MSON%20Specification.md#62-keywords) (nor reserved keywords).

I am happy to change it here but, frankly this just denotes the type of the section from MSON AST perspective where a section is either description section, section that hold member types, validations, sample member types or default member types.
 
No strong opinion here. However if we would rename `validation` to `validations` I would also run with `samples` and `defaults`. But again, this enum specifies the type of MSON AST Type Section so I tend to stay with singular...