(doc) Remove EOL platforms from installation guide
(doc) Remove EOL platforms from installation guide

Ubuntu 15.10 was EOLed on December 2015.

puppetlabs/puppetserver,puppetlabs/puppet-server,camlow325/puppetserver,er0ck/puppetserver,camlow325/puppetserver,er0ck/puppetserver,puppetlabs/puppetserver,er0ck/puppetserver,camlow325/puppetserver,puppetlabs/puppet-server,camlow325/puppet-server,camlow325/puppet-server,puppetlabs/puppet-server,puppetlabs/puppetserver,camlow325/puppet-server,puppetlabs/puppetserver,er0ck/puppetserver,camlow325/puppetserver