remove credentials if blank, causing the SDK to use ec2 role

[chuck014]

This allows the AWS-SDK to connect if run on an ec2 instance without a key/secret needing to be provided.

[GrahamCampbell]

Every line has cs issues. :/

[jeskew]

Couldn't you just remove the credentials array from the published config file?

[GrahamCampbell]

Yeh, probably. Not sure why this PR is needed?

[chuck014]

Did a vendor:publish edited the config/aws.php file removing the credentials array.
It errors asking for a key.

If you remove it as in the PR it lets the SDK use ec2 instance credentials

[jeskew]

Would it be better to just remove the offending lines from the packaged config file?

[jeremeamia]

Yeah, I'd rather see the change in the config.

[jeskew]

@chuck014 That test failure is due to a language feature used in the InstanceProfileProvider not being supported by HHVM. You can flag HHVM as an allowable failure in the .travis.yml file; take a look at how that's done in the SDK package for an example.