Add detection for Rack framework

[krobertson]

This pull request depends on Rack framework support in vcap: cloudfoundry-attic/vcap#186

This is to add detection of a rack application. Quite simply, if there is a config.ru file in the main directory, it considers it Rack. This check is placed before the sinatra check to ensure it takes precedence. If it saw it as sinatra, the config.ru would be ignored on deploy.Dear Cloud Foundry contributor,

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[wallyqs]

It also looks like Rack support was on @ezmobius branch all along... but code is a bit old now.

ezmobius@63b396e

https://github.com/ezmobius/vcap/commit/9e5692df69e762668d8594f2266142423c966422