Is there a reason you are trying to install such an old version and a beta version at that?
You really should just use the following to install the latest version:
PM> Install-Package Microsoft.AspNet.Web.Optimization
It appears you might have some connection issue with your network. What you can do is create a local Nuget repository as described here.
After that you can download the Nuget site and host it on the local repository you have made.