Figure out what is the appropriate ref for any given MAJOR.MINOR semver

This PR replaces #7.

The version is compared against cakephp/cakephp repository in this order:
 - Check VERSION.txt file on master branch
 - Check if any branch names matches the version
 - Check if any tag names matches the version

Example:
- `CAKE_VERSION=1.3` clones branch 1.3
- `CAKE_VERSION=2.0` clones tag 2.0.6
- `CAKE_VERSION=2.4` clones master branch (as it is the latest stable release)

That prevents us of having to update the travis file every time the master branch changes, or when a branch is removed.
Instead we can just have `2.0` which will be translated to the right ref, 2.0.6 tag in this case.

More information: https://github.com/cakephp/datasources/pull/56/files#r6502358

Also added a `Example` plugin with test and a `.travis.yml` which is very much similar to `.travis.example.yml` except that it does not clones the travis repository.