MT#6975 set flake8 and coverage supported versions

We are having this error (flake8):

> pep8style.input_dir(os.path.relpath(location))
> AttributeError: 'StyleGuide' object has no attribute 'input_dir'

See https://github.com/kmmbvnr/django-jenkins/issues/319 for details

And this error (coverage):

> AttributeError: 'Coverage' object has no attribute '_harvest_data'

Change-Id: I0e2dd23e0fc9c072c85ac0cb19a7b69dcc37f849

requirements/test.txt

@@ -1,7 +1,7 @@
 -r common.txt
 django-nose
 git+https://github.com/linuxmaniac/django-jenkins.git@vseva/nose#egg=django-jenkins
-flake8
+flake8==2.4.1
 pylint
-coverage
+coverage==3.7.1
 mock