Use RSpec 2.x until Travis/Capybara issues resolve

https://travis-ci.org/thoughtbot/suspenders/builds/26602519

    rspec-support-3.0.0/lib/rspec/support/version_checker.rb:28:in
   `raise_too_low_error': You are using capybara 2.1.0. RSpec requires
   version >= 2.2.0. (RSpec::Support::LibraryVersionTooLowError)

https://rubygems.org/gems/capybara-webkit