Whitespace
Remove validation duplication
Use AWS_DEFAULT_REGION environment variable

No need set us-east-1 as default region it's already the default. 
If someone is using another region they can use the default AWS environment variable that gets picked up by the AWS SDK
Remove manual AWS configuration, enjoy default env variable names

If you use the default environment variable names there is no need to explicitly configur AWS.
Add note about IAM and AWS_SESSION_TOKEN

http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/iam-roles-for-amazon-ec2.html
Abort will exit an exitcode, without a stack trace
Add support for use of aws_session_token

BundleCache would previously abort when using session_token instead of access_key combination
Add deprecation warning

Avoid breaking change, notify user of deprecation and set correct environment variables
Fix duplicate gemspec dependancy

RubyGems version ~> 2 breaks with

bundle_cache did not have a valid gemspec.
This prevents bundler from installing bins or native extensions, but that may not affect its functionality.
The validation message from Rubygems was:
  duplicate dependency on bundler (~> 1.3, development), (>= 0) use:
    add_runtime_dependency 'bundler', '~> 1.3', '>= 0'
Merge pull request #11 from mriddle/mr-bundle-cache-improvements

AWS Environment variables