Downgrade python-future required version to 0.15.2

It works fine and this way python-future from Ubuntu 16.04 and 18.04 can be used.

requirements.txt

@@ -6,4 +6,4 @@ setuptools
 # Please keep it as the first item of this list.
 #
 pyserial>=3.0
-future>=0.16.0
+future>=0.15.2