Update mixlib-install to e4a3db4
Update mixlib-install to e4a3db4

We are after an important fix to `install.ps1` that will allow us to 
install `delivery-cli` and `push-jobs-client` on Windows:
https://github.com/chef/mixlib-install/pull/111

chef/omnitruck,chef/opscode-omnitruck,chef/opscode-omnitruck,chef/opscode-omnitruck,chef/omnitruck,chef/omnitruck