Just the removed qt55, which required by capybara-webkit, at least for me...
git clone https://github.com/Eric-Guo/homebrew-qt55
cp Formula/qt\@5.5.rb /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/
brew install qt@5.5
Just remove the file you copy to the homebrew-core.
Must install from bottle, if you try compile from source, it will failed, that's why the installation method is different from a normal brew tap.
export PATH="/usr/local/opt/qt@5.5/bin:$PATH"
gem install capybara-webkit