updating capybara to 2.3.0

sorry, but capybara 2.2 wasn't enough :disappointed_relieved:

```
Capybara::RSpecMatchers::HaveText implements a legacy RSpec matcher
protocol. For the current protocol you should expose the failure messages
via the `failure_message` and `failure_message_when_negated` methods.
(Used from /foo/spec/features/foo_spec.rb:16:in `block (2 levels) in <top (required)>')
```

if it's possible, it would be *awesome* to release the next version as `0.7.1`, so minitest-rails-capybara guys doesn't need to release another version as well.

related:
- [x] https://github.com/jnicklas/capybara/pull/1219
- [x] https://github.com/blowmage/minitest-rails-capybara/pull/28
- [x] https://github.com/wojtekmach/minitest-capybara/pull/11