Default locations are broken

Given a `Berksfile`:

```ruby
chef_api :config
site :opscode

cookbook 'berkshelf-cookbook-fixture', '1.0.0'
```

And `berkshelf-cookbook-fixture` does not exist on the Chef Server.

When running `berks install`

Ridley throws an error that we aren't catching and it doesn't try to download from the community site.

---

## Scenario
```text
Scenario: with a default chef_api(1) and site(2) location with a cookbook source that is not satisfied by the chef_api(1) location
  Given I write to "Berksfile" with:
    """
    chef_api :config
    site :opscode

    cookbook 'berkshelf-cookbook-fixture', '1.0.0'
    """
  And the Chef server does not have the cookbooks:
    | berkshelf-cookbook-fixture | 1.0.0 |
  When I successfully run `berks install`
  Then the output should contain:
    """
    Installing berkshelf-cookbook-fixture (1.0.0) from site: 'http://cookbooks.opscode.com/api/v1/cookbooks'
    """
  And the cookbook store should have the cookbooks:
    | berkshelf-cookbook-fixture | 1.0.0 |
  And the exit status should be 0
```
---

## Stacktrace
```text
Scenario: with a default chef_api(1) and site(2) location with a cookbook source that is not satisfied by the chef_api(1) location # features/default_locations.feature:28
  Given I write to "Berksfile" with:                                                                                               # aruba-0.5.2/lib/aruba/cucumber.rb:31
    """
    chef_api :config
    site :opscode

    cookbook 'berkshelf-cookbook-fixture', '1.0.0'
    """
  And the Chef server does not have the cookbooks:                                                                                 # features/step_definitions/chef_server_steps.rb:3
    | berkshelf-cookbook-fixture | 1.0.0 |
  When I successfully run `berks install`                                                                                          # aruba-0.5.2/lib/aruba/cucumber.rb:71
    Exit status was 47 but expected it to be 0. Output:
    
    Ridley::Errors::ResourceNotFound caused by #<Ridley::Errors::HTTPNotFound: {"error":["Cannot find a cookbook named berkshelf-cookbook-fixture"]}>: {"error":["Cannot find a cookbook named berkshelf-cookbook-fixture"]}
    
     (RSpec::Expectations::ExpectationNotMetError)
    features/default_locations.feature:38:in `When I successfully run `berks install`'
  Then the output should contain:                                                                                                  # aruba-0.5.2/lib/aruba/cucumber.rb:113
    """
    Installing berkshelf-cookbook-fixture (1.0.0) from site: 'http://cookbooks.opscode.com/api/v1/cookbooks'
    """
  And the cookbook store should have the cookbooks:                                                                                # features/step_definitions/filesystem_steps.rb:54
    | berkshelf-cookbook-fixture | 1.0.0 |
  And the exit status should be 0                                                                                                  # aruba-0.5.2/lib/aruba/cucumber.rb:154
```