fix osx bootstrap for new double-conversion directory structure

fix osx bootstrap for new double-conversion directory structure

Summary:
Because https://github.com/google/double-conversion/commit/64702b40a2499702f9157ef015f0da6e006114bf

If this breaks an existing checkout, it is because you have an old version of double-conversion now. Go ahead and `rm -rf` it, or start with a clean folly clone.

Test Plan: Clean repo, `./bootstr<tab>`.

Reviewed By: jsedgwick@fb.com

Subscribers: exa, folly-diffs@, yfeldblum, chalfant

FB internal diff: D2136836

Signature: t1:2136836:1433874889:6f6c6a7bd337e052c088aabf89955e9b21b1d8e1