Revert "Force rlp=0.3.9 to attempt fix build"

This reverts commit 29a7673.

Author: Marcin Seremak

setup.py

@@ -32,7 +32,7 @@ def run_tests(self):
 # requirements
 install_requires = set(x.strip() for x in open('requirements.txt'))
 install_requires_replacements = {
-    'https://github.com/ethereum/pyrlp/tarball/develop': 'rlp==0.3.9',
+    'https://github.com/ethereum/pyrlp/tarball/develop': 'rlp>=0.3.9',
     'https://github.com/ethereum/ethash/tarball/master': 'pyethash'}
 install_requires = [install_requires_replacements.get(r, r) for r in install_requires]