YAML isn't part of the Python standard library, while JSON is.
If we want `setup.py` to dynamically parse the available integration
manifest files in order to populate the extra dependencies, then it's
better to rely on a JSON format for manifest files - the parser is part
of the standard library and it doesn't require the user to install
`pyyaml` before `platypush`.
The plugin now leverages the `sound` plugin for playback, like all other
`tts` plugins now do, instead of an external `media` plugin.
This also removes the need for the `/tts/mimic3/say` endpoint.
Fabio Manganiello