Bump net-http-persistent

Needed fix for drbrain/net-http-persistent#59
Cache Bundler dependencies on Travis CI

- Even after the cache is restored, `bundle install` still needs to hit
  RubyGems.org because there is no Gemfile.lock. There doesn't seem to
  be a way to tell it that everything it needs is right there in the
  `./bundle` directory.

- Can't use `--standalone` due to bundler/bundler#2851

- It's not necessary to manually update RubyGems and Bundler anymore.