Update README.md

Using the original bundler example returned "Could not find gem 'aws-sdk-core (~> 2.0) ruby' in the gems available on this machine." on my Mac.

Running 'gem list aws-sdk-core --remote' against https://rubygems.org returned no results.

Thought this might be a good stopgap?
Update README.md

Simplified after feedback from @trevorrowe.
Merge pull request #8 from billgathen/update_readme

Update README.md