Let python-rapidjson~=0.6.0 (#491)

to be consistent with bigchaindb

Author: Troy McConaghy

setup.py

@@ -15,7 +15,7 @@
     'requests>=2.11.0',
     'cryptoconditions==0.8.0',
     'pysha3~=1.0.2',
-    'python-rapidjson==0.6.0',
+    'python-rapidjson~=0.6.0',
     'python-rapidjson-schema==0.1.1',
 ]