fix test setup, added php 5.6 to test matrix

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | no
| Fixed tickets | 
| License       | MIT
| Doc PR        | 

It seems like there is no output for "show" anymore when using SontaAdminBundle 2.3:
http://cmf.liip.ch/en/admin/cmf/menu/menu/cms/menu/main/show

@rande any tip for how to fix this?
@dbu: hmm maybe this has been broken ever since we removed the default ``__toString()`` method?