Tighten constraint on childprocess gem

Open-ended constraints like this are dangerous. Tighten it up.

Change-Id: I462dd8f318f598f8898bd507e4b9ed847852db46
Reviewed-on: http://gerrit.causes.com/46994
Tested-by: jenkins <jenkins@brigade.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>

Author: sds

overcommit.gemspec

@@ -26,7 +26,7 @@ Gem::Specification.new do |s|
 
   s.required_ruby_version = '>= 1.9.3'
 
-  s.add_dependency             'childprocess', '>= 0.5.1'
+  s.add_dependency             'childprocess', '~> 0.5'
 
   s.add_development_dependency 'image_optim', '~> 0.18.0'
   s.add_development_dependency 'rspec', '~> 3.0'