Make explicit that app is running on ruby 2.0.0-p195.

Patrick's comment on [pull request 48](https://github.com/railsbridge-boston/railsbridge-boston/pull/48#issuecomment-18708461) was that Ruby best practice is to specify which version of Ruby you are using.

This `.ruby-version` meets the versioning convention used by rbenv and rvm.  It specifies that the app is running (and being developed on) ruby 2.0.0-p195.

Either gitignoring the ruby version ([pull request 48](https://github.com/railsbridge-boston/railsbridge-boston/pull/48#issuecomment-18708461)), or specifying it (this request), would be helpful.