Newer rubocop wants Rakefile changes

After going to a newer rubocop (0.21.0), it complains about our standard Rakefile:

```
$ bundle exec rake style
Running RuboCop...
Inspecting 1 file
C

Offenses:

Rakefile:32:41: C: Avoid using {...} for multi-line blocks.
  FoodCritic::Rake::LintTask.new(:chef) { |task| 
                                        ^
Rakefile:32:49: C: Trailing whitespace detected.
  FoodCritic::Rake::LintTask.new(:chef) { |task| 
                                                ^
Rakefile:63:15: C: Use %w or %W for array of words.
task default: ['style', 'unit', 'integration']
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

1 file inspected, 3 offenses detected
RuboCop failed!
```

I bumped the version and fixed the complaints in this commit.

```
$ bundle exec rake style
Running RuboCop...
Inspecting 1 file
.

1 file inspected, no offenses detected
```

Thanks!
Martin