Upgrade gitterpy to 0.1.6

[fabaff]

Description:

• Changelog: https://github.com/MichaelYusko/GitterPy/commits/master
• Also, catch error if room is not joined.

Example entry for configuration.yaml (if applicable):

sensor:
  - platform: gitter
    api_key: !secret gitter

Checklist:

If the code communicates with devices, web services, or third-party tools:

• Local tests with tox run successfully. Your PR cannot be merged unless tests pass
• New dependencies have been added to the REQUIREMENTS variable ([example][ex-requir]).
• New dependencies have been added to requirements_all.txt by running script/gen_requirements_all.py.