Sync s3 for deployment

#### What's the plan?
Add s3 sync and deployment configuration and instructions.

#### What's this PR do?
Adds middleman_s3-sync gem with config options in config.rb, adds in a 404 page, adds robots.txt to disallow robots, and adds s3 sync and deployment instructions to the project README.

#### How should this be manually tested?
Follow the README instructions and check that there aren't any errors and files are updated as they should be. You can also visit http://anniversary.texastribune.org.s3-website-us-east-1.amazonaws.com/ to check out the site.

#### What are the relevant tickets?
https://basecamp.com/1866004/projects/1175549/todos/124278441

#### Next steps?
Deploy as needed. When we're ready to change where this points, we'll have to go into s3 and do that, as well as remove the s3 bucket policy restrictions.

#### Smells?
Not really.

#### TODOs:

#### Has the relevant documentation/wiki been updated?
README updated.

#### Technical debt note