Gemfile (:test, :development) specifies Rails 3.x

Without this specification, rails 2.3 was being installed by default.

```
$ ruby --version ruby 1.9.3p385 (2013-02-06 revision 39114)
[x86_64-darwin12.2.0]

$ gem --version 2.0.3

$ bundle --version Bundler version 1.3.3
```