Support Sprockets 3.0

[agrobbin]

A lot changed between version 2 and 3, specifically Hike was dropped entirely as a dependency. That of course means that sprockets_environment.send(:trail) doesn't exist anymore, so the entire reset_sprockets_caches! was going to fail in some spectacular fashion.

Dropping into a pry session to determine what sprockets_environment actually is at within reset_sprockets_caches! point revealed that it was a Sprockets::CachedEnvironment. Eventually, after following all of the code paths within CachedEnvironment, Environment, Base, and the various mixed in modules, you end up coming back to Sprockets::CachedEnvironment and a few instance variables within it, @stats, @entries, and @uris.

These are very similar to the instance variables used in our original reset_sprockets_caches! monkey-patch. @entries is a Hash of directory keys which map to an Array of nested files/directories. @stats is a Hash of all files and directories and their File::Stat counterparts. @uris is a Hash of file URIs and their matching Sprockets::Asset objects.

In the end this turns out not to be such a difficult change. We still have to drill into Sprockets::CachedEnvironment to manipulate its instance variables, but it's actually simpler than before.

[aprescott]

👍 looks good to me!