Fixed "gem build" warning

$ gem build mediawiki_ruby_api_client.gemspec
WARNING:  pessimistic dependency on rest-client (~> 1.6.7) may be
overly strict
  if rest-client is semantically versioned, use:
    add_runtime_dependency 'rest-client', '~> 1.6', '>= 1.6.7'
WARNING:  See http://guides.rubygems.org/specification-reference/ for
help
  Successfully built RubyGem
  Name: mediawiki_ruby_api_client
  Version: 0.0.1
  File: mediawiki_ruby_api_client-0.0.1.gem
Create article via API

Paired with Chris McMahon and Jeff Hall.
Short usage instructions