Remove rails/actioncable Travis badge

[ci skip]

Since Action Cable is getting integrated into rails/rails, the Travis
badge for rails/actioncable is not needed anymore.

Also following the standard of every other README to have `--` rather
than `-` to separating the name of the library from its description.

actioncable/README.md

-# Action Cable – Integrated WebSockets for Rails
-[![Build Status](https://travis-ci.org/rails/actioncable.svg)](https://travis-ci.org/rails/actioncable)
+# Action Cable –- Integrated WebSockets for Rails
 
 Action Cable seamlessly integrates WebSockets with the rest of your Rails application.
 It allows for real-time features to be written in Ruby in the same style