[travis] Add Slack channel notifications.
[travis] Add Slack channel notifications.

openbizgit/phonegap-docs,phonegap/phonegap-docs,jcolinmurray/phonegap-docs,jcolinmurray/phonegap-docs,openbizgit/phonegap-docs,barnyp/phonegap-docs,phonegap/phonegap-docs,barnyp/phonegap-docs,phonegap/phonegap-docs