fix issues with astroid and pylint dpkg packages

[joachimmetz]

Reported by @Onager

Setting up python-astroid (1.4.4-2ppa1~wily) ...
  File "/usr/lib/python2.7/dist-packages/astroid/tests/testdata/python3/data/module2.py", line 100
    print('bonjour', file=stream)
                         ^
SyntaxError: invalid syntax

dpkg: error processing package python-astroid (--configure):
 subprocess installed post-installation script returned error exit status 101
dpkg: dependency problems prevent configuration of python-pylint:
 python-pylint depends on python-astroid; however:
  Package python-astroid is not configured yet.

Work-around

.PHONY: override_dh_python2
override_dh_python2:
    dh_python2 -V 2.7 setup.py

[joachimmetz]

https://codereview.appspot.com/306920043/

[joachimmetz]

Issue merged into #83