Fix KnpLabs/Gaufrette version

| Q                                 | A
| ---------------------------- | ---
| Bug fix?                       | yes
| New feature?              | no
| BC breaks?                 | no
| Tests pass?                | yes
| Scenarios pass?         | n/a
| Checkstyle issues?     | n/a
| Changelog updated?  | n/a
| Fixed tickets               | n/a
| Doc PR                       | n/a

Since Akeneo works only with Gaufrette `v0.1.7` and not with other versions from `0.1.x-dev` you get `Error: Call to undefined method Gaufrette\Filesystem::listKeys()` on imports, this should be stated as direct dependency in the PIM.