Fix bot output when it's to a user

[awentzonline]

This seems to return a dict rather than a string. Looks like there was a similar fix for group messaging here: https://github.com/slackapi/python-rtmbot/commit/bba41f43d99dd5dd3ed513e69291ac5953946629#diff-933761be909a15a181706d770d010687

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[aoberoi]

@awentzonline thanks for the contribution! two things:

1. we'll need you to sign the CLA before we can merge this.

2. travis is complaining about core.py:7:1: F401 'json' imported but unused. i think we just need to remove the import json in that file. would you mind doing that please?