Fix "ruby-debug" listed more than once "warning"

Move "ruby-debug" to a common "platform" section to fix the warning about it being listed multiple times (during "bundle").

```
Your Gemfile lists the gem ruby-debug (= 0.10.4) 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.
```