Add Slack notifications to Travis config

.travis.yml

@@ -1,7 +1,14 @@
 language: ruby
 rvm:
-  - 2.5
+- 2.5
 script:
-  - make -B _site
+- make -B _site
-  - if ! make test; then make test-warning; exit 16; fi
+- if ! make test; then make test-warning; exit 16; fi
 cache: bundler
+notifications:
+  slack:
+    if: branch = master OR branch =~ ^.*slack.*$
+    rooms:
+      - secure: FN4trzyyUdtZ6DXj4G3A7YnhqkPyqi8jN9BIdA6S6D2eJEH1gPbZUfbqAW87LKckKZ9iYB5KDF5IuELhR3vv1DeHRqm3MKRU91JVBDT7EXbF86Z1J/JMJxNOcoWdCilcgpnm9NCkMAPFC9h6Y3txYhaLWLsnDaWlvehBsJg+Ql4=
+    on_success: change
+    on_failure: always