bump rake
bump rake

Fixes a deprecation warning on Ruby 2.7.0

```
$ bundle exec rake
/home/user/.rbenv/versions/2.7.0/lib/ruby/gems/2.7.0/gems/rake-10.5.0/lib/rake/application.rb:381: warning: deprecated Object#=~ is called on Proc; it always returns nil
...
```

See https://github.com/ruby/rake/issues/308

whitequark/ast