Fixes #104: Constraint::check() uses wrong indefinite article in ...

...error message to express the type constraint

Type::check() used a wrong indefinte article on occasion.

This has been fixed by providing a map of types and their wording so that
the correct indefinite article is used, always.

- https://github.com/justinrainbow/json-schema/pull/104
- https://github.com/composer/composer/issues/1616