Update dependency rsa to v4 (#358)

diff --git a/setup.py b/setup.py
index 4e87a72..7149981 100644
--- a/setup.py
+++ b/setup.py
@@ -21,7 +21,7 @@
 DEPENDENCIES = (
     'cachetools>=2.0.0,<3.0',
     'pyasn1-modules>=0.2.1',
-    'rsa>=3.1.4,<4.0',
+    'rsa>=3.1.4,<4.1',
     'setuptools>=40.3.0',
     'six>=1.9.0',
 )