Use SQS => Lambda instead of dispatcher

[austinbyers]

to: @ryandeivert cc: @airbnb/binaryalert-maintainers resolves: #75 resolves: #126

Background

AWS finally added SQS as a Lambda event source!! Lambda will automatically poll the queue, invoke the target function, and delete the message once processed successfully.

This allows us to get rid of the dispatcher Lambda function completely and simplify lots of other logic.

Changes

• Remove the dispatcher Lambda function
• Invoke the analyzer and the downloader via SQS events

Testing

• Deployed to a test account both with and without the downloader

[coveralls]

Coverage decreased (-0.5%) to 92.042% when pulling b8dbcf2786594d64f631abb1458e01b1ec8b24c1 on austin-delete-dispatcher into 6dfcaaf25c2c411176b048cb05a7a75f155e13c2 on master.