Added fallback to default credential provider chain.

It is probably best to use the DefaultCredentialProviderChain for authentication as it allows use of environment variables, IAM roles and credential files.  I would take out the BasicAWSCredentials provider completely but have left it in for back compatibility.
see http://docs.aws.amazon.com/AWSSdkDocsJava/latest/DeveloperGuide/credentials.html

Had to remove the licence file mapping as it is no longer present in the latest SDK

Also increased the version of clojure to 1.6.0 and aws sdk to 1.8.9.1.

I increased the version of the jar to 0.5.0 since I haven't tested that the latest SDK works for all methods.