move six from install_requires to test_require

Signed-off-by: Igor Gnatenko <i.gnatenko.brain@gmail.com>

setup.py

@@ -22,15 +22,14 @@
         'sampledatahelper': ['static/*', 'l10n/names/*',]
     },
     install_requires=[
-        'six',
         'django',
         'sampledata',
     ],
     setup_requires = [
         'versiontools >= 1.8',
     ],
     test_suite = 'nose.collector',
-    tests_require = ['nose >= 1.3.0', 'django >= 1.7.10'],
+    tests_require = ['nose >= 1.3.0', 'django >= 1.7.10', 'six'],
     classifiers = [
         "Programming Language :: Python",
         'Development Status :: 4 - Beta',