[elasticsearch] Support ES 5.x
[elasticsearch] Support ES 5.x

Also, allow the check to correctly handle pre-release versions of
elasticsearch (e.g. `5.0.0-rc1`)

[olivier.vielpeau@datadoghq.com: Simplified the logic a bit]

Wattpad/dd-agent,Wattpad/dd-agent,cberry777/dd-agent,takus/dd-agent,manolama/dd-agent,jyogi/purvar-agent,cberry777/dd-agent,takus/dd-agent,jyogi/purvar-agent,manolama/dd-agent,jyogi/purvar-agent,takus/dd-agent,Wattpad/dd-agent,manolama/dd-agent,manolama/dd-agent,Wattpad/dd-agent,jyogi/purvar-agent,cberry777/dd-agent,Wattpad/dd-agent,takus/dd-agent,cberry777/dd-agent,manolama/dd-agent,cberry777/dd-agent,jyogi/purvar-agent,takus/dd-agent