Update dep of ChildProcess to 0.4.0

This fixes some critical bugs with Windows
core: add output/detail methods to Ui, prefix with arrows
core: colorize VM output for each VM in a command
core: align text and make sure its all lined up with prefixes
providers/virtualbox: many `detail` level output
providers/virtualbox: more detailed output
Fix tests for new UI methods
core: convert more output to `output` method
core: Ui color should be red on errors, yellow on warnings
hosts/gentoo: fix nonstandard path to systemctl command
core: extensive tests for Vagrant::UI classes
core: make output white for now
Merge pull request #2858 from timgus/gentoo_systemctl

hosts/gentoo: fix nonstandard path to systemctl command
Update CHANGELOG
hosts/gentoo: more robust check for systemctl path on Gentoo
guests/coreos: restart etcd after configuring networks [GH-2852]
Fixed: dup string if frozen when formatting output for machine.
Merge pull request #2861 from morrizon/machine_readable_issue

commands/box-list: dup string if frozen when formatting output for machine.
website/docs: clarify upgrade breakage is a bug
Update CHANGELOG