Fix utf8 weirdness on tests

This is an weird issue that makes the spec with charset fail on TravisCI. This lead to some investigation that the combination of Linux + mysql2 gem does not allow to search/index accented chars completely. Forcing this param on the indices file it fixes the issue.

Also I confirmed that without this option my app on production on a EC2 machine is not able to search accented chars.

Maybe we should also update the documentation?

Fixes #629