Pin cryptoconditions==0.7.2 in setup.py, for now (#477)

This is to get ReadTheDocs working, or at least partly working. To understand the reasoning, see bigchaindb/bigchaindb#2512

Author: Troy McConaghy

setup.py

@@ -13,7 +13,7 @@
 
 install_requires = [
     'requests>=2.11.0',
-    'cryptoconditions~=0.7.2',
+    'cryptoconditions==0.7.2',
     'pysha3~=1.0.2',
     'python-rapidjson==0.6.0',
     'python-rapidjson-schema==0.1.1',