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
Enforce Ruby version

The pessimistic operator makes it possible to install Suspenders on Ruby
2.1.2 but then run Suspenders and have it not bundle because the Gemfile
is restricted to Ruby 2.1.1. This change enforces an exact Ruby version.