Replace pycrypto package

It seems to have trouble installing on my system, and the project has been dead for quite a while and is exploitable. We should pick a new crypto library to use. https://github.com/dlitz/pycrypto/issues/287

It doesn't appear that we actually use it very heavily -- it's mentioned in working with encrypted device data (https://github.com/iotile/coretools/blob/3df8511941b7f22b0d255db7358f19e8b001a9b0/README.md#working-with-encrypted-device-data) and present in a single import (https://github.com/iotile/coretools/blob/8050a048b4048a82914f0f980e791e32dc6d0db4/iotilecore/iotile/core/hw/auth/env_auth_provider.py#L175) so moving over should be easy.