Log Message:
Update ruby-contracts to 0.16.0.

## v0.16.0

- **Support for Ruby 1.8 has been discontinued** - [Corey \ 
Farwell](https://github.com/frewsxcv) \ 
[#256](https://github.com/egonSchiele/contracts.ruby/pull/256)
- Enhancement: Add a `Contracts::Attrs` module containing attribute w/ contracts \ 
utilities - [Corey Farwell](https://github.com/frewsxcv) \ 
[#255](https://github.com/egonSchiele/contracts.ruby/pull/255)
- Bugfix: Fix StrictHash contract for extra keys - [Maciej \ 
Malecki](https://github.com/smt116) \ 
[#254](https://github.com/egonSchiele/contracts.ruby/pull/254)

## v0.15.0
- Bugfix: Func contract's return value isn't enforced with blocks - [Piotr \ 
Szmielew](https://github.com/esse) \ 
[#251](https://github.com/egonSchiele/contracts.ruby/pull/251)
- Bugfx: Fix contracts used in AR-models - [Gert Goet](https://github.com/eval) \ 
[#237](https://github.com/egonSchiele/contracts.ruby/pull/237)