Fix for Ruby 2.0 and 2.1, which no longer support lambdas on rescue blocks.

I was playing with MCC Admin and Blueprint, making calls to MAudit that should have timed out.  Instead of timing out gracefully, I was seeing:
```
TypeError: class or module required for rescue clause
	from /mnt/mccadmin/bundle/ruby/2.0.0/bundler/gems/rack-cache-15390785b682/lib/rack/cache/context.rb:364:in `rescue in send_with_retries'
```
It looks like the "feature" of being able to pass a function to a rescue clause was actually unintentional?  At least that's what it seems like [from this thread](https://www.ruby-forum.com/topic/4419633), and a comment on [this post](http://ablogaboutcode.com/2011/09/21/dynamic-rescue-clause-in-ruby/).

Here are the results of running the bacon tests.  Prior to this change, running on ruby-2.0.0-p481:
```
TypeError: class or module required for rescue clause
  /Users/clabrunda/md_rails/rack-cache/lib/rack/cache/context.rb:205:in `rescue in validate_with_retries_and_stale_cache_failover': Rack::Cache::Context - allows an exception to be raised when a connection error occurs while revalidating a cached entry if fault_tolerant is set to true but the per-request is false
 ... <full stack trace snipped>
269 specifications (927 requirements), 0 failures, 10 errors
```
With this change:
```
269 specifications (977 requirements), 0 failures, 0 errors
```