Default worker count to the number of processors

This allows sprockets-derailleur to be dropped into a rails 4 project
and "just work". You can also override the worker count with an
environment variable.
Do not override initialize

It differs in different versions of sprockets. In rails 4 it can be
called with only one parameter.
Merge pull request #10 from substantial/default-worker-count

Default worker count to the number of processors