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.
Fix Ruby 1.8.7 warning about regex syntax.

Noticed this when running against Ruby 1.8.7, there were two repeated
warnings based on improper escaped brackets:

- spec/rspec/rails/matchers/route_to_spec.rb:114:
  > warning: regexp has invalid interval

- spec/rspec/rails/matchers/route_to_spec.rb:114:
  > warning: regexp has `}' without escape
updating feature docs to reflect 3.0 final release
Merge pull request #1092 from plentz/patch-2

updating feature docs to reflect 3.0 final release
Merge pull request #1089 from rspec/fix-ruby-warnings

Fix warnings and deprecations notices
updating feature docs to reflect capybara 2.2.0 dependency

change made in 8254ddc10c8e34912300ca2c8df275cc5132d1ba
updating to 2.3.0 which fixes some warnings