Deprecate alerting multiple channels

Some CIs report to two channels, and some others report to only one.
This makes it consistent. Only alert channel should be alerted.

.travis.yml

@@ -527,7 +527,6 @@ notifications:
 
   slack:
     rooms:
-      - secure: i1GLETSKye85ea6dGNA3MxI/5myChmMFiZtBd5C69xK+s1sBFqEgOSbaSf9KHc0CYrHVyNhQMaZRruieV7xS+6Pfs0Zvxf1DO6QQTWC2KhkqwFDLvZncAzjoyASdR90hbr+iRPOngQ+HJuE94zemALAwEqNAinzA74PMiJXktqY= # ruby:<token>#commits
       - secure: ah7UEHBvncXT7bM5mvYIQAO+tIyV/wl7nXLb7wQD16dO2v8Gragy0mWjB79Q09hrrMGmp6H9bCDpdGS80boIA5EHaHoG4QaP0i9bsSt8U2AMWgZtfyIgQKJ4H2kXkGlrjO+AXTgnIkP7LNjdgAVUUTGQPb26T3QmoN2Splt+fIQ= # ruby:<token>#alerts
     on_pull_requests: false
     on_success: change

appveyor.yml

@@ -144,25 +144,3 @@ notifications:
     on_build_success: false
     on_build_failure: true
     on_build_status_changed: true
-  - provider: Webhook
-    method: POST
-    url:
-      secure: iMINHMS0nZabaDsxN9omRDsekxzVvAAzEq5ev7lN6vb+gUETT+rbDKLGxBxBpEpxlnPlLdzroIJ+DTKlwfJA8VkGawTn9EXNsucH0OkSf2M= # AppVeyor CI
-    body: >-
-      {{^isPullRequest}}
-        {
-          "attachments": [
-            {
-              "title": "Build {{projectName}} {{buildVersion}} {{status}}",
-              "title_link": "{{buildUrl}}",
-              "fallback": "AppVeyor Build {{projectName}} {{buildVersion}} {{status}}",
-              "text": "Commit <{{commitUrl}}|{{commitId}}> by {{commitAuthor}} on {{commitDate}}: _{{commitMessage}}_",
-              "color": "{{#passed}}good{{/passed}}{{#failed}}danger{{/failed}}"
-            }
-          ],
-          "channel": "#commits"
-        }
-      {{/isPullRequest}}
-    on_build_success: false
-    on_build_failure: true
-    on_build_status_changed: true