Rationale: installing thin/puma needs a working toolchain and various
libraries installed. Failing that, "gem install" will abort with an
error which doesn't give much hints to people not familiar with ruby.
Webrick is much slower, but it's pure ruby and doesn't depend on any
other gem. Sinatra will pick up what is available, in this order of
preference: thin, puma, mongrel, webrick.

This frequently confuses first time users.

Sinatra uses `:bind` to specify the interface to bind to: http://www.sinatrarb.com/configuration.html#bind---server-hostname-or-ip-address

The readme suggested the config file controlled which riemann host to
talk to; this is a total lie.