missing require time lib for Time.parse is used in EC2Provider, the test passed because there is an example required the time lib
Fix bug: default credential provider hides sub credential provider's fetching
credentials error.

The default credential provider will try to rescue sub provider's
MissingCredentialsError for looking up the right credential provider.
However, the way using 'rescue' keyword is not correct, and it
actually rescued all the StandardError raised from sub provider's
credentials method.
Merge pull request #147 from ThoughtWorksStudios/credentials_provider_bug

Fix bug: default credential provider hides sub credential provider's fetching credentials error
Merge pull request #145 from ThoughtWorksStudios/master

Add missing require time lib in credential_providers.rb
Fix undefined method 'version_id' during multipart upload
Fixed an issue with the stubbed responses for S3::Client#complete_multipart_upload.

A number of tests that should have failed were passing because the
simulate response for #complete_multipart upload was puttin in a nil
value for :version_id.  The version id actually should only be present
when not-nil.