Ignore generated file from RuboCop

[pocke]

rubocop-rubycw has been introduced by #113, but it records offenses in my local.

$ bundle exec rake rubocop
Inspecting 44 files
...........................C................

Offenses:

lib/ruby/signature/parser.rb:2988:1: C: Rubycw/Rubycw: mismatched indentations at 'end' with 'module' at 9
    end   # module Signature
^
lib/ruby/signature/parser.rb:2989:1: C: Rubycw/Rubycw: mismatched indentations at 'end' with 'module' at 8
  end   # module Ruby
^

44 files inspected, 2 offenses detected

So this pull request suppresses the warnings by adding the generated file to the exclude list.

---

By the way, I'm not sure why the cop does not add offense on Travis CI. The builds are green on the master and other pull requests.

[hanachin]

I fixed it.
ruby/racc@b0431d4#diff-691c91a72009483e9d5c04fae13e9953