Fix PyPI long description rendering (#1739)

Currently the rendering of gensim's reStructuredText description on PyPI
is broken[1][2], because some characters aren't escaped properly[3]:

>reStructuredText uses backslashes ("\") to override the special meaning
>given to markup characters and get the literal characters themselves.
>To get a literal backslash, use an escaped backslash ("\\").

[1]: https://pypi.python.org/pypi/gensim/3.1.0/
[2]: https://pypi.org/project/gensim/3.1.0/
[3]: http://docutils.sourceforge.net/docs/user/rst/quickref.html#escaping

setup.py

@@ -196,7 +196,7 @@ def finalize_options(self):
 
   @inproceedings{rehurek_lrec,
         title = {{Software Framework for Topic Modelling with Large Corpora}},
-        author = {Radim {\v R}eh{\r u}{\v r}ek and Petr Sojka},
+        author = {Radim {\\v R}eh{\\r u}{\\v r}ek and Petr Sojka},
         booktitle = {{Proceedings of the LREC 2010 Workshop on New
              Challenges for NLP Frameworks}},
         pages = {45--50},