Create master snapshot builds

[ycombinator]

Currently, when grunt build is run, it creates versioned snapshot builds like so in the target folder:

kibana-5.0.0-snapshot-darwin-x64.tar.gz
kibana-5.0.0-snapshot-linux-x64.tar.gz
...

These are then published to S3 and become available via download.elastic.co.

It would be nice to additionally have master snapshot builds that are named like so:

kibana-master-snapshot-darwin-x64.tar.gz
kibana-master-snapshot-linux-x64.tar.gz
...

This way there's a permalink to get the latest build of master, regardless of version.

Possibly related to #6941.