Update rspec to 2.99.0.beta1
Convert specs to latest rspec syntax
Update rspec to 3.0.0.beta1
Fix specs on Config#[]=? methods
Improve MemoryFile specs:

* Nest into ZAssets module;
* Use explicit subject instead of implicit one.
Improve Server specs:

* Improve syntax;
* Refactor some examples
Improve an expectation in SprocketsEnv spec