Set ruby version for CircleCI

Created by: calrrox

Hi, excuse me for return to discuss this.

On the issue #253 it was concluded that it was not necessary to add to "circle.yml" the version of ruby.

But I think so, because the ".ruby-version" file is used by "RVM" to declare "gemset" for example, and the name of this "gemset" is user preference.

So I think it is best set to "circle.yml" version of ruby and add to ".gitignore" the file ".ruby version" like another projects.