adjust slack notifications
adjust slack notifications

Problem:
Attempt to send successful travis build notices to the main dev channel for the repo and failed build notices to a general event channel resulted in no build notices being sent.

Analysis:
Testing shows that `.travis.yml` can accept multiple rooms, but not multiple `on_success`/`on_failure` flags. Contributers who enable travis with github and then enable this repo receive emails on change and on failure by default. To keep noise to a minimum while preserving actionable messages slack notifications seem best to be configured for the event ("gitlab") channel.

Solution:
- Modified slack configuration to be more explicit.
- Removed section of `.travis.yml` that attempted to send success only messages.

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