[GH-2932] Disable brittle test

The AWS security group tests have been very unstable with 2 failures on
Travis per build. The majority is the mocked version of
`describe_security_groups` not working.

We have decided to disable it until someone makes time to figure out the
bug since it is affecting every build.

See https://travis-ci.org/fog/fog/jobs/25614603#L1105