fix build task for creating distributable versions of kibana

[spalger]

Fixes #4591

With the new server, the build system was broken and needs to be fixed. The server currently runs in a production-like state by simply running ./bin/kibana but does not generate the distributable packages or trim down the files.

This must include compiling the server's source code with babel.

[skearns64]

Since this one is complete, is it safe to re-add the nightly download links to the readme? This issue is referenced from the readme where there used to be a link: https://github.com/elastic/kibana

[skearns64]

self-reply: #4733 is already open for that.. whoops!