Remove farmers' markets from DB setup task

This is necessary for the Ohana API Admin specs to pass.

Farmers' Markets were specific to SMC. It was just an additional data source we decided to add, but it's not part of the OpenReferral schema.

There are still some references to markets, namely within Kind, which we will deal with later. The question is whether or not the Kind field would be useful to other cities since the assumption is that all OpenReferral data is of the Human Services kind.
Update Ruby version to 2.1.1
Update README with Ruby version support
Remove Sentry gem

Sentry should be removed from the project because it's not an essential gem. It's just a tool for getting notified about errors and helps you troubleshoot them. It's up to the redeployment team what debugging and reporting tools they want to use.
Update README with note about Mongoid not working in Rails 4
Remove unused area gem
Comment out rack-throttle gem

It's only needed for rate limiting, which is not being used currently
Remove rails_admin

Rails_Admin was originally used when we first started experimenting with admin interfaces. We decided that having a separate app for the Admin interface would allow for more flexibility and easier customization. So now that ohana-api-admin exists, there's no need for Rails_Admin in this repo at this time.
Remove Rails_Admin