WIP 164: Bug Fixes

WIP: https://assemblymade.com/helpful/wips/164

This PR fixes:

* Bug referenced in WIP 164. By default User factories no longer create a membership, user_with_membership is available should you need that functionality. Ultimately we need to flesh out multiple membership support.

* Bug seen in recent travis builds: https://travis-ci.org/austinylin/helpful-web/builds/15567534#L156. The fix here is to manually namespace the BillingsController#Webhook method into "/webhooks". Need to have a discussion re: moving BillingsController#Webhook into a separate controller under Webhooks::Chargify.

cc: @drewblas, @chrislloyd