Fix "unrecognized cop" warning in brew style

[apjanke]

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Fixes an "unrecognized cop" warning by splitting out the RSpec-specific cop configuration to its own file that's only used when brew style --rspec is called.

Before:

$ brew style                                                                                                                                  master
Warning: unrecognized cop Rspec/ExpectActual found in .rubocop.yml
Warning: unrecognized cop Rspec/ExpectActual found in .rubocop.yml
Inspecting 657 files
..............................................................................................................................................................................

After:

$ brew style                                                                                                               fix-rspec-rubocop-warning
Inspecting 657 files
.......................................................................................................................................................................

Should probably be merged before doing any more rspec-rubocop configuration, to avoid even more warnings.

[MikeMcQuaid]

Nice! Thanks again @apjanke!