Fix cryptography dependency (#46)

The `cryptography` package is never required directly by the
`nexmo` package. Correct `install_requires` so that nexmo
requires the `PyJWT[crypto]` dependency (which is how
`PyJWT` spells its optional dependency on `cryptography`
in its `extras_require`). Now if `PyJWT` ever changes the
underlying dependency it uses to implement its optional
`crypto` functionality away from `cryptography` to some
other library, nexmo will pull the replacement transitive
dependency rather than continuing to pull the now-wrong
transitive dependency, leaving things in a broken state.

`cryptography` *is* directly required by the tests, so add it
to `tests_require`.

Author: jab

setup.py

@@ -15,7 +15,8 @@
       license='MIT',
       packages=['nexmo'],
       platforms=['any'],
-      install_requires=['requests', 'PyJWT', 'cryptography'],
+      install_requires=['requests', 'PyJWT[crypto]'],
+      tests_require=['cryptography'],
       classifiers=[
           'Programming Language :: Python',
           'Programming Language :: Python :: 2',