Support only 5.6 and later.
In the future:

• We keep compatibility with older ES version as long as it’s practical
• When some changes require to drop support for an older ES version, we do it in a new minor of Search
• We advertise some minors as “LTS”, to allow people to stay on older versions of ES while still receiving critical patches.

[Openshift version of ES is tracked here: https://github.com/openshift/origin-aggregated-logging/blob/master/elasticsearch/Dockerfile ]