dependency has been renamed

In
https://github.com/bbatsov/rubocop/commit/ff167d8f202baf7a68955db0aaf0dc29afb7e7ee
the Rubocop class was renamed to RuboCop with no backwards compat
handling.
update rubocop style config

Ignore the Rakefile for hash rockets, since it's run under a Ruby 2.1
interpreter.
Choose to ignore whitespace between argument definition and value, as
spacing helps us read blocks of parameters.