Update Travis CI Configuration

The previous configuration seems to have an issue:

> WARNING: Using Objective-C testing without specifying a scheme and either a workspace or a project is deprecated. [#25175295](https://travis-ci.org/herinkc/HTPressableButton/builds/25175295)

Hope this helps:

* Use a build only script since the `HTPressableButtonTests` are not implemented
* Add a build status badge