Never use `then` for multi-line `if/unless`.

See also: https://github.com/styleguide/ruby
tweak

* Fix `_calc_reqs_per_child`
* Use `do ... end` for multi-line blocks. See also https://github.com/styleguide/ruby
* Remove useless `;`
Merge pull request #2 from kamipo/master

Follow Ruby coding style guide.