Update site content for Rails4/RSpec3

[onebree]

I think it is important to still keep current best practices, but label older ones as for RSpec 2. Many teams and online classes still use RSpec2, and it is important to maintain compatibility.

• Clarify whether a best practice is for only RSpec2 or RSpec3
• Update all reference links (Upgraded link to RSpec 3.2 docs #139, Update Rails Test Prescriptions link to latest edition #141)
• Replace Spork recommendation with Spring (Rails 4)
• Update gems in project, and recommended gems on site
• Research other best practices site, and include (and attribute) content if needed
• Translate all changes into other languages
• Write guidelines in second-person (Guidance phrased as personal opinion #142)
• Banner explaining the use of older Rails/RSpec versions and site maintenance (Update Rails Test Prescriptions link to latest edition #141 (comment))