Log Message:
Update ruby-capybara to 2.15.1.

# Version 2.15.1

Release date: 2017-08-04

### Fixed

*  `attach_file` with no extension/MIME type when using the `:rack_test` driver
   [Thomas Walpole]

# Version 2.15.0

Release date: 2017-08-04

### Added

*  `sibling` and `ancestor` finders added [Thomas Walpole]
*  Added ability to pass options to registered servers when setting
*  Added basic built-in driver registrations `:selenium_chrome` and
   `:selenium_chrome_headless` [Thomas Walpole]
*  Add `and_then` to Capybara RSpec matchers which behaves like the previous
   `and` compounder. [Thomas Walpole]
*  Compound RSpec expectations with Capybara matchers now run both matchers
   inside a retry loop rather than waiting for one to pass/fail before
   checking the second.  Will make `#or` more performant and confirm both
   conditions are true "simultaneously" for `and`.  [Thomas Walpole] If you
   still want the
*  Default filter values are now included in error descriptions [Thomas Walpole]
*  Add `Session#refresh` [Thomas Walpole]
*  Loosened restrictions on where `Session#within_window` can be called from
   [Thomas Walpole]
*  Switched from `mime-types` dependency to `mini_mime` [Jason Frey]