Log Message:
Update ruby-rack-cache to 1.5.1.

## 1.5.0

  * only catch StandardError and not Exception

## 1.4.3

  * After overriding the REQUEST_METHOD, store the original request method in \ 
"rack.methodoverride.original_method"

## 1.4.1

  * Ignore invalid Expires date as per RFC

## 1.4.0

  * Not invalidating the cache for preflight CORS request

## 1.3.1 / October 2015

  * Support Ruby 1.9

## 1.3 / Octorber 2015

  * Ruby 2.0 only

  * Gracefully degrade when cache store goes offline

  * allow_reload/revalidate is not enabled by default

  * Make Rack::Cache multithread friendly