Fix a CI failure with `FileUtils.rmdir` behavior changed Ruby 2.5
Fix a CI failure with `FileUtils.rmdir` behavior changed Ruby 2.5

Currently Travis CI has failed to build ruby-head.

https://travis-ci.org/bbatsov/rubocop/jobs/294862639#L682-L700

The error log is as follows.

```console
  1) RuboCop::ResultCache cached result that was saved with no command
  line option when no option is given when a symlink is present in the
  cache location and symlink attack protection is disabled permits
  caching and prints no warning
     Failure/Error: FileUtils.rmdir(attack_target_dir)

     Errno::ENOTEMPTY:
       Directory not empty @ dir_s_rmdir - /tmp/d20171030-6334-1pf27sw
     # ./spec/rubocop/result_cache_spec.rb:97:in `block (5 levels) in <top (required)>'
     # ./lib/rubocop/rspec/shared_contexts.rb:27:in `block (4 levels) in <top (required)>'
     # ./lib/rubocop/rspec/shared_contexts.rb:26:in `chdir'
     # ./lib/rubocop/rspec/shared_contexts.rb:26:in `block (3 levels) in <top (required)>'
     # ./lib/rubocop/rspec/shared_contexts.rb:8:in `block (2 levels) in <top (required)>'
```

In Ruby 2.5 the behavior of `FileUtils.rmdir` changes to non-empty directory.

- https://github.com/ruby/ruby/commit/2635984cf2b9b7632f8b35afa2f29d219aba104a
- https://github.com/ruby/ruby/commit/1241d59cefbe52fa13ad7bcfabe9dca5182c06d6

## Ruby 2.4.2

```console
% rm -rf /tmp/foo && ruby -v -rfileutils -e
'FileUtils.mkdir("/tmp/foo"); FileUtils.touch("/tmp/foo/bar.txt"); FileUtils.rmdir("/tmp/foo")'
ruby 2.4.2p198 (2017-09-14 revision 59899) [x86_64-darwin13]
% ls /tmp/foo
bar.txt
```

## Ruby 2.5.0-dev (2017-10-30 trunk 60580)

```console
% rm -rf /tmp/foo && ruby -v -rfileutils -e
'FileUtils.mkdir("/tmp/foo"); FileUtils.touch("/tmp/foo/bar.txt"); FileUtils.rmdir("/tmp/foo")'
  ruby 2.5.0dev (2017-10-30 trunk 60580) [x86_64-darwin13]
  Traceback (most recent call last):
        4: from -e:1:in `<main>'
        3: from /Users/koic/.rbenv/versions/2.5.0-dev/lib/ruby/2.5.0/fileutils.rb:247:in `rmdir'
        2: from /Users/koic/.rbenv/versions/2.5.0-dev/lib/ruby/2.5.0/fileutils.rb:247:in `each'
        1: from /Users/koic/.rbenv/versions/2.5.0-dev/lib/ruby/2.5.0/fileutils.rb:248:in `block in rmdir'
```

Since the file was not deleted even in Ruby 2.4 or lower, it seems that
it was probably working differently than intended.

This commit uses `FileUtils.rm_rf` instead of `FileUtils.rmdir`.

mikegee/rubocop,vergenzt/rubocop,jmks/rubocop,maxjacobson/rubocop,tejasbubane/rubocop,sue445/rubocop,smakagon/rubocop,bquorning/rubocop,vergenzt/rubocop,panthomakos/rubocop,bquorning/rubocop,urbanautomaton/rubocop,tdeo/rubocop,smakagon/rubocop,akihiro17/rubocop,panthomakos/rubocop,meganemura/rubocop,petehamilton/rubocop,deivid-rodriguez/rubocop,rrosenblum/rubocop,jmks/rubocop,akihiro17/rubocop,maxjacobson/rubocop,deivid-rodriguez/rubocop,sue445/rubocop,tejasbubane/rubocop,scottmatthewman/rubocop,pocke/rubocop,bbatsov/rubocop,mikegee/rubocop,mikegee/rubocop,haziqhafizuddin/rubocop,tdeo/rubocop,deivid-rodriguez/rubocop,jfelchner/rubocop,meganemura/rubocop,tdeo/rubocop,petehamilton/rubocop,bquorning/rubocop,akihiro17/rubocop,maxjacobson/rubocop,palkan/rubocop,rrosenblum/rubocop,vergenzt/rubocop,petehamilton/rubocop,urbanautomaton/rubocop,palkan/rubocop,pocke/rubocop,meganemura/rubocop,jfelchner/rubocop,rrosenblum/rubocop,palkan/rubocop,scottmatthewman/rubocop,haziqhafizuddin/rubocop,sue445/rubocop,jmks/rubocop,bbatsov/rubocop,jfelchner/rubocop,panthomakos/rubocop,tejasbubane/rubocop