Fix Build (RSpec 2 and formatters change)

This PR does two things:

* set RSpec to 2.14 (since there are some non-RSpec 3 specs)
* use `eval` instead of `const_get` for formatters lookup.

After setting the RSpec version, I noticed that some formatter tests were failing because the `const_get` call was finding the `Text` module instead of the `Tailor::Formatters:Text` class. `Text` appears to be from another gem and is used [here](https://github.com/turboladen/tailor/blob/master/lib/tailor/configuration.rb#L224).