Raise ElasticSearch timeouts from 2.0s to 5.0s

In [#83](https://github.com/alphagov/rummager/pull/83) a default timeout of
2.0s was introduced on requests to ElasticSearch. This resulted in a
number of 5xx errors and as a result the timeout in production was
raised to 5.0s - this PR is simply to document that fact.

- If we can resolve the root cause of why those requests took more than
  2.0s, then this PR can be closed and the default will remain 2.0s
- If we cannot resolve the root cause of those requests taking too long,
  then this PR should be merged before Rummager is next deployed to
  Production.