Update generator_containers.py

Use `generator_function.__name__` for Python2 compatibility instead of `generator_function.__qualname__`.

See issue WebexCommunity#57.

Update dependencies

Update generator_containers.py

Use `generator_function.__name__` instead of `generator_function.__qualname__` for Python2 compatibility.

PEP8

Fix datetime properties

Object properties should either return their value or `None` if no value is present.

Update Travis config

Travis isn't supporting Python 3.7 yet

Prepare for release

PEP8

Set minimum retry-after wait time to 1 second

We believe Cisco Spark is returning a `Retry-After` of `0` most probably due to a millisecond rounding issue,  which would mean that the `Retry-After` time should really fall somewhere between `0 < t < 1`.

To account for the expectation that some amount of wait time was expected before the request should be retried, change any `Retry-After` responses of `0` seconds to `1` second to provide a minimum wait time for requests to be retried.

Ensure SparkRateLimitError.retry_after is a non-negative int

Fix bug number 2 associated with issue WebexCommunity#52 :

Ensure that the SparkRateLimitError.retry_after attribute is always a non-negative int, and then (in the automated rate-limit handling code in restsession.py) don't test for the validitiy of the retry_after attribute - make sure it is good and then use it.