Fixed several "gem build" warnings

Also converted remaining single quotes to double quotes.

$ gem build mediawiki_selenium.gemspec
WARNING:  pessimistic dependency on cucumber (~> 1.3.10) may be overly
strict
  if cucumber is semantically versioned, use:
    add_runtime_dependency 'cucumber', '~> 1.3', '>= 1.3.10'
WARNING:  pessimistic dependency on headless (~> 1.0.1) may be overly
strict
  if headless is semantically versioned, use:
    add_runtime_dependency 'headless', '~> 1.0', '>= 1.0.1'
WARNING:  pessimistic dependency on json (~> 1.8.1) may be overly strict
  if json is semantically versioned, use:
    add_runtime_dependency 'json', '~> 1.8', '>= 1.8.1'
WARNING:  pessimistic dependency on net-http-persistent (~> 2.9.1) may
be overly strict
  if net-http-persistent is semantically versioned, use:
    add_runtime_dependency 'net-http-persistent', '~> 2.9', '>= 2.9.1'
WARNING:  pessimistic dependency on page-object (~> 0.9.5) may be
overly strict
  if page-object is semantically versioned, use:
    add_runtime_dependency 'page-object', '~> 0.9', '>= 0.9.5'
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:  pessimistic dependency on rspec-expectations (~> 2.14.4) may
be overly strict
  if rspec-expectations is semantically versioned, use:
    add_runtime_dependency 'rspec-expectations', '~> 2.14', '>= 2.14.4'
WARNING:  pessimistic dependency on syntax (~> 1.2.0) may be overly
strict
  if syntax is semantically versioned, use:
    add_runtime_dependency 'syntax', '~> 1.2', '>= 1.2.0'
WARNING:  See http://guides.rubygems.org/specification-reference/ for
help
  Successfully built RubyGem
  Name: mediawiki_selenium
  Version: 0.2.0
  File: mediawiki_selenium-0.2.0.gem

Change-Id: Ifb7d3f84d47afed203fd0f127b3b049195b77765