chore: remove gitter integration (#907)

No change to logic.

This removes gitter integration with the project. I don't monitor gitter
right now, and I've never noticed the integration to be particularly
useful otherwise.

Test: N/A

.travis.yml

@@ -21,12 +21,5 @@ script:
 after_success:
   - npm run codecov -- -f coverage/lcov.info
 
-# Gitter
 notifications:
   email: false
-  webhooks:
-    urls:
-      - https://webhooks.gitter.im/e/743dc6b83b6dd05111ee
-    on_success: change  # options: [always|never|change] default: always
-    on_failure: always  # options: [always|never|change] default: always
-    on_start: never     # options: [always|never|change] default: always