Update ruby-childprocess to 0.6.3.

### Version 0.6.3 / 2017-03-24

See beta release notes.

### Version 0.6.3.beta.1 / 2017-03-10

* Bug fix: Fixed child process creation problems on Windows 7 when a child was declared as a leader.

### Version 0.6.2 / 2017-02-25

* Bug fix: Fixed a potentially broken edge case that could occur on older 32-bit OSX systems.

### Version 0.6.1 / 2017-01-22

* Bug fix: Fixed a dependency that was accidentally declared as a runtime
  dependency instead of a development dependency.

### Version 0.6.0 / 2017-01-22

* Support for Ruby 2.4 added

(taca)