Fix minimum required version of cryptography

setup.py

@@ -95,7 +95,7 @@ setup(
     namespace_packages=['ligo'],
     packages=find_packages(),
     classifiers=CLASSIFIERS,
-    install_requires=['future>=0.15.0', 'six>=1.9.0', 'cryptography>=1.7.2'],
+    install_requires=['future>=0.15.0', 'six>=1.9.0', 'cryptography>=2.5'],
     setup_requires=SETUP_REQUIRES,
     tests_require=tests_require,
     package_data={