update net-http-persistent to v2.8

This pull request updates the vendored net-http-persitent library to latest as discussed #2274.

With this newer version of net-http-persistent bundler uses the no_proxy environment variable and skips the proxy for excluded host.

I tried to come up with a test for the no_proxy feature but couldn't figure out how to modify the singelton `Net::HTTP::Persistent` class that is created in `Bundler::Fetcher` to inject the no_proxy and proxy settings.