chore(travis): add gitter config

Author: IgorMinar

.travis.yml

@@ -30,3 +30,11 @@ script:
 
 after_script:
   - ./scripts/travis/print_logs.sh
+
+notifications:
+  webhooks:
+    urls:
+      - https://webhooks.gitter.im/e/d2120f3f2bb39a4531b2
+    on_success: change  # options: [always|never|change] default: always
+    on_failure: always  # options: [always|never|change] default: always
+    on_start: false     # default: false