Get wicked_pdf tests running on Travis

.travis.yml

before_script:
  - sudo apt-get install -y wkhtmltopdf
  - "export DISPLAY=:99.0"
  - "sh -e /etc/init.d/xvfb start"

env: WKHTMLTOPDF_BIN=/usr/bin/wkhtmltopdf

wicked_pdf.rb

# This file lives in config/initializers

WickedPdf.config = {
  :exe_path => ENV['WKHTMLTOPDF_BIN'] || '/usr/local/bin/wkhtmltopdf'
}