Up required gradle version in contributing instructions

[softwaredoug]

ES now requires gradle 4.3, change contributing.md to match

• Have you signed the contributor license agreement?
• Have you followed the contributor guidelines?
• If submitting code, have you built your formula locally prior to submission with gradle check?
• If submitting code, is your pull request against master? Unless there is a good reason otherwise, we prefer pull requests against master and will backport as needed.
• If submitting code, have you checked that your submission is for an OS that we support?
• If you are submitting this code for a class then read our policy for that.

[elasticmachine]

Since this is a community submitted pull request, a Jenkins build has not been kicked off automatically. Can an Elastic organization member please verify the contents of this patch and then kick off a build manually?

[elasticmachine]

Since this is a community submitted pull request, a Jenkins build has not been kicked off automatically. Can an Elastic organization member please verify the contents of this patch and then kick off a build manually?

[softwaredoug]

FTR my company (opensource connections, github org o19s) has signed the CLA

[DaveCTurner]

@elasticmachine please test this

[DaveCTurner]

LGTM

[DaveCTurner]

6.0 -> 7872aaa
6.1 -> 1e53212
6.x -> 3b74b9a
master -> 507aac4