Release a new version of capybara-webkit

[jamesRaybould]

Hello,

Would it please be possible to release the latest version of master to RubyGems?

[twalpole]

@jferris @mhoran Quite a few people have recently had issues where bundler has been selecting capybara-webkit 1.1.0 to use with the latest Capybara release, which of course causes all sorts of issues. Would it be possible for you to release a 1.15.0 from master in the near future?

[jferris]

I updated the changelog, updated the version, tagged, and relased v1.15: https://rubygems.org/gems/capybara-webkit/versions/1.15.0