Fix development dependencies gems requirements

The previous development dependencies were strange, like requiring Rails and Rake versions ```0.x.y```. Also, RSpec ```3.0.0rc1``` breaks the specs, so I fixed it to ```3.0.0beta2```.

To test the error behavior, assuming you're running Bash and RVM, first remove all the installed gems with (some errors might show, but it's ok):
```for x in `gem list --no-versions`; do gem uninstall $x -a -x -I; done```
and then install the gems with bundler:
```bundle install```
then run the specs with ```rake```, and the issue will appear.