Slack notifications (#740)

* Slack notifications

* get that formatting back in

* Using env var on travis's web ui

* Update .travis.yml

* Making it like IRC was

.travis.yml

@@ -8,7 +8,7 @@ matrix:
     fast_finish: true
     allow_failures:
       - php: 7.3
-      
+
 services:
   - mysql
 
@@ -41,3 +41,9 @@ script:
 
 after_success:
   - bash <(curl -s https://codecov.io/bash)
+
+notifications:
+  slack:
+    on_success: change
+    on_failure: always
+    secure: $SLACK_NOTIFICATION_KEY