Support Capybara 2.3 and 2.4

[dtognazzini]

[dtognazzini]

The "release notes":
https://github.com/jnicklas/capybara/blob/master/History.md

Capybara does not follow semantic versioning.

Breaking changes in Capybara 2.3:
New window management API [Andrey Botalov]
others?

Breaking changes in Capybara 2.4:
TBD

[dtognazzini]

Will be released in 1.5 a la #169