Merge pull request #1147 from benoit-pierre/fix_travis_2.6_job

fix `test_pip_upgrade_from_source` test on Python 2.6

setuptools/tests/test_virtualenv.py

 import glob
 import os
+import sys
 
 from pytest import yield_fixture
 from pytest_fixture_config import yield_requires_config
@@ -39,6 +40,9 @@ def test_pip_upgrade_from_source(virtualenv):
     Check pip can upgrade setuptools from source.
     """
     dist_dir = virtualenv.workspace
+    if sys.version_info < (2, 7):
+        # Python 2.6 support was dropped in wheel 0.30.0.
+        virtualenv.run('pip install -U "wheel<0.30.0"')
     # Generate source distribution / wheel.
     virtualenv.run(' && '.join((
         'cd {source}',