Don't profile 10 slowest examples in rspec

The output was annoying
Pin to Sprockets 2.x

The development branch of Sprockets has some breaking changes that I'd
like to avoid hitting by accident.
Add initial version of processor code + tests
Fix TODO by deleting some duplicate checks

Sprockets::Environment#find_asset will return nil when an asset doesn't
exist, so we don't need to duplicate this check ourselves.
Add SimpleCov

Start tracking coverage of my miniscule amount of code.
Fix bad escaping in documentation URL
Fix base path used when resolving assets

Sprockets::Context#pathname is the full path to the current file being
processed, not the directory containing it.