Removed shelly-dependencies gem.

It is not necessery, because Gemfile already contain
'rake' and 'thin' gems. This is not a Rails application
so there is no need to configure static assets and
compiling assets on request.
Removed shelly gem from Gemfile.

It is not recommended to keep it in Gemfile.
Increased number of thins from 4 to 6.

Small virtual server on shelly should handle it (in fact
it should handle more, but we can start from this point).