Improve ruby gemspec

[Jens-G]

WARNING:  open-ended dependency on logger (>= 0) is not recommended
  use a bounded requirement, such as "~> x.y"
WARNING:  open-ended dependency on rack (>= 2.2.23, development) is not recommended
  if rack is semantically versioned, use:
    add_development_dependency "rack", "~> 2.2", ">= 2.2.23"
WARNING:  See https://guides.rubygems.org/specification-reference/ for help

[kpumuk]

I would honestly ignore the warnings, they are aimed at applications and not libraries. With this change people will not be able to use Thrift gem with modern popular libraries, like Sidekiq.