Fix bug in check for index existing.

Elasticsearch 0.90+ raises a 404 instead of returning an empty response
on the _aliases endpoint when an alias doesn't exist; this change makes
index.exists? handle this response, and in turn allows
rummager:migrate_index work when there aren't existing indexes
Fix real_name call when index doesn't exist.

This moves the check for a 404 on the _alias endpoint from the exists
method to real_name, since various bits of code expect that to work.
Merge pull request #188 from alphagov/fix_index_exists

Fix bug in check for index existing.