Update Core Tech: Rack Gem

[angelatan2]

This is a follow-up story from Ignite Sprint's goal.

• Understand if methods were deprecated by gem owners.
• Find new methods that are of value
• Instrument those new methods
• Update our multiverse to dynamically pull the latest version so that we do not have to manually update each gem version number manually.

[workato-integration]

https://issues.newrelic.com/browse/NEWRELIC-3340

[fallwith]

Understand if methods were deprecated by gem owners.

We instrument URLMap and Builder. Neither of these have been deprecated. Builder#run has been updated to accept a block, however, and we will need to conditionally (if Rack >= v3.0.0) update our own instrumentation to handle a block as well.

Find new methods that are of value

and

Instrument those new methods

From looking over the added list, there isn't anything new on the same level as Builder or URLMap.

Update our multiverse to dynamically pull the latest version so that we do not have to manually update each gem version number manually.

This has already been done for Rack.