fixed simple deprecation warnings (issue #61)

[zmk5]

Fixed two simple deprecation warnings as outlined in #61. The main deprecation warning, however, requires a bit more work since they have an issue with the yield function used within the async def.

[zmk5]

The build failure is because of the python 2.7 build.

[clalancette]

This looks generally good to me; thanks for the update.

@wjwwood Do we still care about Python 2 compatibility in this repository? If no, then I'll follow up with a PR disabling it in travis. If yes, we may have to do a bit more work here to make it compatible with both Python 2 and Python 3.

[wjwwood]

I'm fine with dropping Python 2 support now, but we have to stop release on Xenial at the same time.

[clalancette]

I'm fine with dropping Python 2 support now, but we have to stop release on Xenial at the same time.

Out of curiousity, why? Xenial has python 3.5, which should be new enough to support the code here. (I don't have a strong opinion on it, I'm just wondering)

[wjwwood]

It is used by ROS 1 users in Xenial, and they use it with python2. We could instead only release new versions on Xenial as python3, leaving the python2 version behind, but that's a bit confusing that the python3 version suddenly is higher than python2.

[zmk5]

I added conditionals to the deprecation so once xenial is dropped we can remove them. I also found out getfullargspec() now returns 7 values instead of 4 according to the docs.