Travis CI: enable setuptools entry points

The new plugin architecture depends on setuptools entry points being
properly registered. These entry points are defined in a `setup.py`
file, as the `entrypoints` parameter, but they are only effective when
they become an `entry_points.txt` file inside a
`<your_project>.egg_info` directory.

This can happen in two ways:

1) installing the package, that is, running `$ python setup.py
install`

2) enabling the development mode, that is, running `$ python setup.py
develop`.

The approach chosen here is the latter, because it's faster, doesn't
require a full install and points to the checked out source code tree.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

.travis.yml

@@ -20,5 +20,6 @@ script:
     - inspekt style
     - ./selftests/cyclical_deps avocado
     - ./selftests/modules_boundaries
+    - python setup.py develop
     - ./selftests/run
     - ./selftests/check_tmp_dirs