Add version bounds to development dependencies

[dgraham]

Silences warnings during gem build.

$ gem build charlock_holmes.gemspec
WARNING:  open-ended dependency on rake-compiler (>= 0.7.5, development) is not recommended
  if rake-compiler is semantically versioned, use:
    add_development_dependency 'rake-compiler', '~> 0.7', '>= 0.7.5'
WARNING:  open-ended dependency on minitest (>= 0, development) is not recommended
  if minitest is semantically versioned, use:
    add_development_dependency 'minitest', '~> 0'
WARNING:  open-ended dependency on chardet (>= 0, development) is not recommended
  if chardet is semantically versioned, use:
    add_development_dependency 'chardet', '~> 0'
WARNING:  See http://guides.rubygems.org/specification-reference/ for help

This also prevents issues with a future Minitest 6 release.

$ bundle exec rake test
DEPRECATED: Use assert_nil if expecting nil from /Users/david/Code/charlock_holmes/test/encoding_detector_test.rb:147. This will fail in Minitest 6.

[brianmario]

Thanks!!