Issue/320 index name based current tenant

[simonireilly]

Example PR which shows the changes made to the algoliasearch-rails gem.

1. Fix flaky spec by deleting all indices between runs
2. Add a per_tenant method which puts a prefix on the index_name - Tested with Penthouse & Apartment

Sample application with Apartment and index per model per tenant (subdomain) -> https://github.com/simonireilly/algolia-playground