Update childprocess dependency to 0.5.6 minimum

This is necessary as of d9453db so tests pass when we call `exit?` on a
detached process.

Author: sds

CHANGELOG.md

@@ -49,6 +49,7 @@
   with that version, copy the [default configuration from 0.23.0](https://github.com/brigade/overcommit/blob/9f03e9c82b385d375a836ca7146b117dbde5c822/config/default.yml)
 * Update `ScssLint` pre-commit hook to properly handle special exit code that
   signals all files were filtered by exclusions (new as of `scss-lint` 0.36.0)
+* Update `childprocess` dependency to minimum 0.5.6
 
 ## 0.23.0
 

overcommit.gemspec

@@ -26,7 +26,7 @@ Gem::Specification.new do |s|
 
   s.required_ruby_version = '>= 1.9.3'
 
-  s.add_dependency             'childprocess', '~> 0.5.0'
+  s.add_dependency             'childprocess', '~> 0.5.6'
 
   s.add_development_dependency 'rspec', '~> 3.0'
   s.add_development_dependency 'travis', '~> 1.7'