Require Ruby 1.9.3 via Gemfile

* Supported by Heroku https://devcenter.heroku.com/articles/ruby-versions
Add poltergeist and database_cleaner for JS acceptance specs

* Use poltergeist because of lower dependency overhead.

* For JS specs, the tests run in a different thread than the app,
  so we use DatabaseCleaner's truncation strategy to ensure
  the DB state is clean between specs.

  See "Transactions and database setup" in Capybara README:
  https://github.com/jnicklas/capybara/blob/master/README.md
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