Upgrade slacker to 0.12.0

[fabaff]

Description:

Commit log: https://github.com/os/slacker/commits/master

Example entry for configuration.yaml (if applicable):

notify:
  - name : slack
    platform: slack
    api_key: !secret slack_api
    default_channel: '#ha'

Tested with the following content for Service data:

{"message": "The sun is {% if is_state('sun.sun', 'above_horizon') %}up{% else %}down{% endif %}!"}

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in home-assistant.io

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

• New dependencies have been added to the REQUIREMENTS variable (example).
• New dependencies are only imported inside functions that use them (example).
• New or updated dependencies have been added to requirements_all.txt by running script/gen_requirements_all.py.
• New files were added to .coveragerc.

If the code does not interact with devices:

• Tests have been added to verify that the new code works.