[make] allow updating new website version
[make] allow updating new website version

Summary:
`make new-website-version` would fail if run a second time, because it
assumes the version is brand new. Now, it will delete previous traces of
the new version, which allows one to easily update commits upgrading the
website instead of reverting the diff then running `make
new-website-version` again.

Reviewed By: martintrojer

Differential Revision: D24218463

fbshipit-source-id: 61f416677

jvillard/infer,jvillard/infer,facebook/infer,jvillard/infer,jvillard/infer,facebook/infer,jvillard/infer,facebook/infer,facebook/infer,facebook/infer,jvillard/infer,facebook/infer,jvillard/infer,facebook/infer