Update beberlei/assert to ~2.0

Previously discussed in [this issue][1] and [this comment][2]

We're currently using Broadway, but also want to use the new Assertion features (`Assert\that()` and the lazy assertion) introduced in beberlei/assert version 2.0. This updates the library without backwards compatibility issues.

Resolves https://github.com/qandidate-labs/broadway/issues/16

What has been done?
- Added the recommended compatibility classes under `Broadway\Assert`, see [here][3]
- Updated usages of `Assert\Assertion` to `Broadway\Assert\Assertion`

No failing tests, one test that has already been marked as skipped is left untouched.

Any thoughts/feedback?

Regards,

Daan.

[1]: https://github.com/qandidate-labs/broadway/issues/16
[2]: https://github.com/qandidate-labs/broadway/pull/14#issuecomment-53543435
[3]: https://github.com/beberlei/assert#your-own-assertion-class