Elasticsearch has an active community and the release cycles are very fast.
Because Elasticsearch depends on many common Java libraries (Lucene, Guice, and Jackson are the most famous ones), the Elasticsearch community tries to keep them updated and fixes bugs that are discovered in them and in Elasticsearch core. The large user base is also source of new ideas and features for improving Elasticsearch use cases.
For these reasons, if it's possible, the best practice is to use the latest available release (usually the more stable one and the less bugs free).
A supported Elasticsearch operative system (Linux/MacOSX/Windows) with a Java JVM 1.8 (the Oracle one is the preferred http://www.oracle.com/technetwork/java/javase/downloads/jdk8-downloads-2133151.html ...