check correct version string for rspec composable

IMO `Capybara::RSpecMatchers` (its name) doesn't make any sense without RSpec-Expectations.

I think this file should either require rspec/expectations or suppose that it's already loaded. Requiring rspec/expectations isn't an option as this file is unconditionally required by `capybara/rspec` that may be used in environments where people use e.g. rspec-core + minitest-assertions.

Ideally, there should be `require 'capybara/rspec/matchers' if defined?(RSpec::Expectations)` (and also `if` when including it into config) inside `capybara/rspec` but that won't be backwards compatible.

IMO the best option is to not require rspec-expectations here. Although there is no harm in doing what you do here that looks like unnecessary code to me.