Fixed warning message displayed while building the gem

$ gem build mediawiki-selenium.gemspec
WARNING:  open-ended dependency on cucumber (>= 0) is not recommended
  if cucumber is semantically versioned, use:
    add_runtime_dependency 'cucumber', '~> 0'
WARNING:  open-ended dependency on json (>= 0) is not recommended
  if json is semantically versioned, use:
    add_runtime_dependency 'json', '~> 0'
WARNING:  open-ended dependency on net-http-persistent (>= 0) is not
recommended
  if net-http-persistent is semantically versioned, use:
    add_runtime_dependency 'net-http-persistent', '~> 0'
WARNING:  open-ended dependency on page-object (>= 0) is not recommended
  if page-object is semantically versioned, use:
    add_runtime_dependency 'page-object', '~> 0'
WARNING:  open-ended dependency on rest-client (>= 0) is not recommended
  if rest-client is semantically versioned, use:
    add_runtime_dependency 'rest-client', '~> 0'
WARNING:  open-ended dependency on rspec-expectations (>= 0) is not
recommended
  if rspec-expectations is semantically versioned, use:
    add_runtime_dependency 'rspec-expectations', '~> 0'
WARNING:  open-ended dependency on syntax (>= 0) is not recommended
  if syntax is semantically versioned, use:
    add_runtime_dependency 'syntax', '~> 0'
WARNING:  See http://guides.rubygems.org/specification-reference/ for
help
  Successfully built RubyGem
  Name: mediawiki-selenium
  Version: 0.1.16
  File: mediawiki-selenium-0.1.16.gem

Change-Id: I43ffef1d6bcdfbebfad22dd593894be8739d9806