Mention in our Readme that the minimal Python 3 supported version is 3.5 (#608)

Author: ccordoba12

README.rst

@@ -10,7 +10,7 @@ Python Language Server
 .. image:: https://img.shields.io/github/license/palantir/python-language-server.svg
      :target: https://github.com/palantir/python-language-server/blob/master/LICENSE
 
-A Python 2.7 and 3.4+ implementation of the `Language Server Protocol`_.
+A Python 2.7 and 3.5+ implementation of the `Language Server Protocol`_.
 
 Installation
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