Add -f flag so curl will fail when kibana states it's not ready

[mdiehm]

tested and worked with 6.5.1 - Fixes #66

from the curl manpage:

   -f, --fail
          (HTTP) Fail silently (no output at all) on server errors. This is mostly done to better enable
          scripts etc to better deal with failed attempts. In normal cases when an HTTP server fails  to
          deliver a document, it returns an HTML document stating so (which often also describes why and
          more). This flag will prevent curl from outputting that and return error 22.

          This method is not fail-safe and there are occasions where non-successful response codes  will
          slip through, especially when authentication is involved (response codes 401 and 407).