add travis build status

Author: JazzCore

README.rst

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 Python-PDFKit: HTML to PDF wrapper
 ==================================
 
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+.. image:: https://travis-ci.org/JazzCore/python-pdfkit.png?branch=master
+        :target : https://travis-ci.org/JazzCore/python-pdfkit
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 Python 2 and 3 wrapper for wkhtmltopdf utility to convert HTML to PDF using Webkit.
 
 This is adapted version of `ruby PDFKit <https://github.com/pdfkit/pdfkit>`_ library, so big thanks to them!