Bump poltergeist from 1.17.0 to 1.18.1

[dependabot-preview]

Bumps poltergeist from 1.17.0 to 1.18.1.

Changelog

Sourced from poltergeist's changelog.

1.18.1

Bug Fixes

• Fixed #current_url should return correct url when on 'about:blank' (Thomas Walpole)[Issue #927]

1.18.0

Features

• User agent and other headers persist across windows (Tony Novak)[Issue #915]
• Capybara 3 support (Thomas Walpole)
• Support configuring PhantomJS page setting when initializing driver (Thomas Walpole)
• Support for mouse click with offset and button presses (Thomas Walpole)

Bug Fixes

• Fixed page source potentially blank based on page load timing (Thomas Walpole)

Commits

• 1254f34 Prepare for 1.18.1 release
• dd96a31 Merge pull request #928 from teampoltergeist/about_blank
• 825909f current_url should be about:blank when on about:blank - Fix Issue # 927
• d3e0d8b Prepare for 1.18.0 release
• e555ccd Merge pull request #925 from teampoltergeist/datalist_tests
• c9b34bc Merge pull request #924 from teampoltergeist/page_settings
• 8cb7210 Skip datalist tests because PJS doesn't support datalist
• 1d07fa4 Support overriding the PJS page settings when initializing driver - http://ph...
• 895c90e Merge pull request #916 from afn/master
• c0f6660 Merge pull request #923 from teampoltergeist/travis_defaults_pjs_211
• Additional commits viewable in compare view

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