Update README.md

Author: uralbash

README.md

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 [![Build Status](https://travis-ci.org/ITCase/sqlalchemy_mptt.svg?branch=master)](https://travis-ci.org/ITCase/sqlalchemy_mptt)
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+[![PyPI](http://img.shields.io/pypi/dm/sqlalchemy_mptt.svg)](https://pypi.python.org/pypi/sqlalchemy_mptt)
 
 Library for implementing Modified Preorder Tree Traversal with your SQLAlchemy Models and working with trees of Model instances, like django-mptt.