(SERVER-13) Fix for file descriptor leak during report processing

This commit bumps up the dependency that Puppet Server has on
puppetlabs/http-client to version 0.2.8.  The http-client 0.2.8 release
contains a fix for a file descriptor leak which previously occurred on
every HTTP client request.  Previously, this leak would be encountered
by Puppet Server for each client request it would make to a report
processor, e.g., PuppetDB.