handle :native_or_unimrcp renderer in #execute_command

Adding a new switch for :native_or_unimrcp allows us to play audiofiles
natively and TTS via unimrcp for ASR question types. Previously, only
DTMF question types handled :native_or_unimrcp appropriately

Files affected:

- lib/punchblock/translator/asterisk/call.rb
- spec/punchblock/translator/asterisk/call_spec.rb