fix python2.6 issue with the new test

Author: RonnyPfannschmidt

testing/test_regressions.py

@@ -53,8 +53,8 @@ def test_pip_egg_info(tmpdir, monkeypatch):
 def test_pip_download(tmpdir, monkeypatch):
     monkeypatch.chdir(tmpdir)
     subprocess.check_call([
-        sys.executable, '-m',
-        'pip', 'download', 'lz4==0.9.0',
+        sys.executable, '-c',
+        'import pip;pip.main()', 'download', 'lz4==0.9.0',
     ])