Changed the default signature version for Amazon EC2 to sigv4.
Now producing two gem artifacts, `aws-sdk` and `aws-sdk-v1`.

Adding `aws-sdk-v1` gem, making it possible for v1 and v2
of the AWS SDK for Ruby to be loaded in the same Ruby
process.

Also updated the README with information about using proper version
constraints in Gemfile.
Updated Gemfile to use the aws-sdk-v1 gemspec.