Update Ruby version noted in README to 2.1.1

Ruby version changed to 2.1.1 in this commit, whereas README lists Ruby version as 2.0.0 https://github.com/rails/rails-dev-box/commit/0f2143cd3cc2ab8ec596956ab46e1b34de0724f6
Merge pull request #75 from eliotsykes/patch-1

Update Ruby version noted in README to 2.1.1