Fix duplicate gem warning in Gemfile

Group `:development` and `:test` both had `rr` and `rake` pegged at the same
version, and Bundler was complaining:

```
$ bundle 
Your Gemfile lists the gem rake (>= 0.8.7) more than once.
You should probably keep only one of them.
While it's not a problem now, it could cause errors if you change the version of just one of them later.
Your Gemfile lists the gem rr (~> 1.0.2) more than once.
You should probably keep only one of them.
While it's not a problem now, it could cause errors if you change the version of just one of them later.
```

This commit breaks them out into their own block. Personally I'd lump everything into the `group :development, :test` block, but I may be unaware of reasons to keep them separate.