Fixed fog require error

Hi @slyness 

Running into this issue with latest Fog `v1.24.0`

```
[5] pry(main)> con = Fog::DNS.new(aws)
NameError: uninitialized constant Fog::Parsers::Base
from /Users/arnauddelabarre/.rvm/gems/ruby-1.9.3-p545@chef/gems/fog-1.24.0/lib/fog/aws/parsers/dns/health_check.rb:5:in `<module:AWS>'
```

We should require `fog` directly to get around future refactors of the gem.

Thanks!