Fixed "gem build" warning

$ gem build mediawiki_selenium.gemspec
WARNING:  pessimistic dependency on mediawiki_api (~> 0.1.3) may be
overly strict
  if mediawiki_api is semantically versioned, use:
    add_runtime_dependency 'mediawiki_api', '~> 0.1', '>= 0.1.3'
WARNING:  See http://guides.rubygems.org/specification-reference/ for
help
  Successfully built RubyGem
  Name: mediawiki_selenium
  Version: 0.2.26
  File: mediawiki_selenium-0.2.26.gem

Change-Id: I2a261d35a3686b6790a24ae1a6774041a365a8a8