Don't attempt to install ruby-switch in ubuntu 14.04 onwards

ruby-switch is gone from 14.04 onwards. I don't know too much about chef so if you need me to add any tests / change anything just point me in the right direction.
I'd prefer this logic to go in the [attributes file](https://github.com/mojolingo/brightbox-ruby-cookbook/blob/develop/attributes/default.rb), along with the declaration of the default for this attribute. I'd also prefer to call it `'install_ruby_switch'`, rather than `'force_switch'`.