readme: update version in header

We don't need to explicitly set a version of Tarantool in header because
it may confuse Tarantool users. Python connector supports Tarantool with
versions below and upper version 1.6.0. Section below describes more
details about compatibility of Tarantool and connector versions.

Author: ligurio

README.rst

@@ -1,5 +1,5 @@
-Python driver for Tarantool 1.6
-===============================
+Python driver for Tarantool
+===========================
 
 This package is a pure-python client library for `Tarantool`_.