Move development dependencies from gemspec to Gemfile

[dentarg]

I noticed warnings when build the gem before pushing it to RubyGems.org:

cd ./sinatra-contrib && gem build sinatra-contrib.gemspec
WARNING:  description and summary are identical
WARNING:  open-ended dependency on multi_json (>= 0) is not recommended
  use a bounded requirement, such as '~> x.y'
WARNING:  open-ended dependency on asciidoctor (>= 0, development) is not recommended
  use a bounded requirement, such as '~> x.y'
WARNING:  open-ended dependency on builder (>= 0, development) is not recommended
  use a bounded requirement, such as '~> x.y'
WARNING:  open-ended dependency on erubi (>= 0, development) is not recommended
  use a bounded requirement, such as '~> x.y'
WARNING:  open-ended dependency on haml (>= 0, development) is not recommended
  use a bounded requirement, such as '~> x.y'
WARNING:  open-ended dependency on liquid (>= 0, development) is not recommended
  use a bounded requirement, such as '~> x.y'
WARNING:  open-ended dependency on markaby (>= 0, development) is not recommended
  use a bounded requirement, such as '~> x.y'
WARNING:  open-ended dependency on nokogiri (>= 0, development) is not recommended
  use a bounded requirement, such as '~> x.y'
WARNING:  open-ended dependency on rake (>= 12.3.3, development) is not recommended
  if rake is semantically versioned, use:
    add_development_dependency 'rake', '~> 12.3', '>= 12.3.3'
WARNING:  open-ended dependency on redcarpet (>= 0, development) is not recommended
  use a bounded requirement, such as '~> x.y'
WARNING:  open-ended dependency on slim (>= 0, development) is not recommended
  use a bounded requirement, such as '~> x.y'
WARNING:  See https://guides.rubygems.org/specification-reference/ for help
  Successfully built RubyGem
  Name: sinatra-contrib
  Version: 3.0.6
  File: sinatra-contrib-3.0.6.gem

I think we can make them go away by moving them to the Gemfile

sinatra/sinatra-contrib/sinatra-contrib.gemspec

Lines 46 to 57 in 516ee3b

	s.add_development_dependency 'asciidoctor'
	s.add_development_dependency 'builder'
	s.add_development_dependency 'erubi'
	s.add_development_dependency 'haml'
	s.add_development_dependency 'liquid'
	s.add_development_dependency 'markaby'
	s.add_development_dependency 'nokogiri'
	s.add_development_dependency 'rack-test', '~> 2'
	s.add_development_dependency 'rake', '>= 12.3.3'
	s.add_development_dependency 'redcarpet'
	s.add_development_dependency 'rspec', '~> 3'
	s.add_development_dependency 'slim'

I think it makes sense to do for sinatra (only rack-test there now) and rack-protection (rack-test and rspec) too