Fixed pinned dependency versions

These can cause real headaches if you have a different version of six or requests on your host machine.

This should fix the ``pkg_resources.DistributionNotFound`` error some people have been getting with the "wagtail start" command.

setup.py

@@ -39,8 +39,8 @@ install_requires = [
     "beautifulsoup4>=4.3.2",
     "html5lib==0.999",
     "Unidecode>=0.04.14",
-    "six==1.7.3",
+    "six>=1.7.0",
-    'requests==2.3.0',
+    'requests>=2.0.0',
 ]