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.

/reviewed-on !20