Test for Capybara::Session definition, rather than Capybara

as Capybara can be defined even when Capbybara::Session is not, in
which case we get:

    NameError:
          uninitialized constant Capybara::Session
          /rspec-html-matchers-0.4.3/lib/rspec-html-matchers.rb:97:in `matches?'