Fix remaining Ruby warnings.

- spec/rspec/rails/minitest_lifecycle_adapter_spec.rb:
  > warning: assigned but unused variable - example

- spec/support/shared_examples.rb
  > warning: assigned but unused variable - spec_file

Ruby 1.9.3 seems to erroneously report the following warning:

- lib/rspec/rails/matchers/have_http_status.rb:
  > warning: assigned but unused variable - _ignored

This is logged on ruby-trunk: https://bugs.ruby-lang.org/issues/9943
Update Capybara to remove matcher deprecations.
Upgrade Aruba gem dependency.

The new version requires an additional, optional, `timeout` argument.
Include this argument in the world shim.