Suppress keyward argument warnings in Ruby 2.7
Suppress keyward argument warnings in Ruby 2.7

This PR suppresses the following keyward argument warnings
when using Ruby 2.7.

```console
% ruby -v
ruby 2.7.0dev (2019-09-12T15:35:29Z master 69acf40b45) [x86_64-darwin17]
% bundle exec rake

(snip)

/Users/koic/src/github.com/cucumber/aruba/lib/aruba/platforms/unix_platform.rb:116:
warning: The last argument is used as the keyword parameter
/Users/koic/.rbenv/versions/2.7.0-dev/lib/ruby/2.7.0/fileutils.rb:605:
warning: for `rm_r' defined here
/Users/koic/src/github.com/cucumber/aruba/lib/aruba/platforms/unix_platform.rb:135:
warning: The last argument is used as the keywo
rd parameter
/Users/koic/.rbenv/versions/2.7.0-dev/lib/ruby/2.7.0/fileutils.rb:1120:
warning: for `touch' defined here
./Users/koic/src/github.com/cucumber/aruba/lib/aruba/platforms/unix_platform.rb:150:
warning: The last argument is used as the keyword parameter
/Users/koic/.rbenv/versions/2.7.0-dev/lib/ruby/2.7.0/fileutils.rb:1012:
warning: for `chmod_R' defined here
```

cf. https://bugs.ruby-lang.org/issues/14183

mvz/aruba,mvz/aruba,cucumber/aruba