Readme refers to deprecated method

[ketumbra]

The readme suggests use of the deprecated TestSuite.to_xml_string() method instead of the new to_xml_report_string() method from #62.

see https://github.com/kyrus/python-junit-xml/blob/master/README.rst#using

[IndraTeja]

Created PR #77 with changes to address this issue. Thank you.