Require pycrypto 2.6+ at the gce.py module level.

This PR adds a `pycrypto>=2.6` requirement at the module level for the GCE inventory script.

This is important because it's very easy to end up in a situation where both 2.0 and 2.6 are installed by the operating system because both CentOS and Ubuntu provide separate _packages_. 2.0 often wins in precedence order, and this module needs to use the 2.6 one.