Fix for Ohai::Exceptions::DependencyNotFound

Fix for the following bug : 

    Ohai::Exceptions::DependencyNotFound: Can not find a plugin for dependency os
    /var/lib/gems/2.0.0/gems/ohai-7.0.2/lib/ohai/system.rb:138:in `require_plugin'
    /var/lib/gems/2.0.0/gems/bunchr-0.1.5/lib/bunchr/ohai.rb:11:in `ohai'
    /var/lib/gems/2.0.0/gems/bunchr-0.1.5/lib/bunchr/utils.rb:32:in `ohai'
    /var/lib/gems/2.0.0/gems/bunchr-0.1.5/lib/bunchr/packages.rb:27:in `initialize'
    [...]

Catch when building sensu with sensu-build.