Update version of flake8-import-order package

Use latest version of flake8-import-order package, like we do in
openstack/ironic

Fix two issues detected by new version.

Change-Id: I3d2f5499f3a863167419469d313e06be79f51841

ironic_lib/tests/base.py

@@ -18,7 +18,6 @@
 import subprocess
 
 import mock
-
 from oslo_concurrency import processutils
 from oslotest import base as test_base
 

ironic_lib/tests/test_disk_partitioner.py

@@ -15,7 +15,6 @@
 
 import eventlet
 import mock
-
 from testtools.matchers import HasLength
 
 from ironic_lib import disk_partitioner

test-requirements.txt

@@ -4,7 +4,7 @@
 
 coverage!=4.4,>=4.0 # Apache-2.0
 eventlet!=0.18.3,!=0.20.1,<0.21.0,>=0.18.2 # MIT
-flake8-import-order==0.11 # LGPLv3
+flake8-import-order>=0.13 # LGPLv3
 hacking>=1.0.0 # Apache-2.0
 mock>=2.0.0 # BSD
 os-testr>=1.0.0 # Apache-2.0