erp5_wendelin: fix WendelinInformationGadget installation
While investigating fixes to https://nexedi.erp5.net/bug_module/20151207-1789A61 I found that it was impossible to reproduce this bug in Wendelin tests due to a bad configuration of how WendelinInformationGadget was installed. Now I can Wendelin tests (still get a different problem though, which is subject for another PR or bug report).
Please review, @Tyagov.