It's useful for us to know, which version of which software
is serving gems. This patch adds an `"X-Powered-By"` response
header with the value of `"geminabox VERSION"`, e.g. `"geminabox 0.6.0"`.
geminabox$ bundle exec rackup
$ curl -I localhost:9292
HTTP/1.1 200 OK
X-Powered-By: geminabox 0.6.0