testing should include stable branch

[benaryorg]

I suggest to add the testing target stable to .travis.yml.

But as this is an administrative decision, I am not going to send a pull-request.

Further targets include:

• beta
• 1.2.0
• 1.3.0
• 1.4.0
• 1.5.0

The version targets as a sort of promise-statement.
As if you said

The library runs on all of these platforms.

And only the incompatible, old platforms are then removed (and new old ones (current stable) added).

This is just a suggestion though.

[benaryorg]

This would also provide some sort of testing for the new versions as this is a commit and only commits are triggering the build.