[wip] Upgrade to RSpec 3 

So far:
- Upgrade from RSpec 2.99 to 3.1
- Replaced .should with RSpec's 3 expect syntax

Todo:

`example_group` is deprecated, therefore https://github.com/Willianvdv/approvals/blob/rspec3/lib/approvals/namers/directory_namer.rb#L15-L17 doesn't work anymore. I'm not sure how to fix this. I could use :full_path, but that returns (when normalized):
```
approvals_namers_directorynamer_uses_non_funky_example_description
```
instead of:
```
approvals_namers_directorynamer/uses_non_funky_example_description
```