Install script should not write partially downloaded binaries

[xzyfer]

Currently the binary download is streamed to disk once a 200 response
has been recieved. When an error occurs during the download a partially
downloaded binary is left on disk. Subsequent installs see the binary
and bail out of re-downloading it. Worse yet those subsequent installs
move the binary into the global cache so even removing node_modules
will not remove the broken binary.

With this patch the binary is only flushed to disk once it has been
fully downloaded.

Fixes #1882
Fixes #1888

[xzyfer]

Interesting the failure results in process.dlopen error message. This could be the root cause for so many of our previous issues.

module.js:598
  return process.dlopen(module, path._makeLong(filename));

[saper]

What about going a bit further - writing to a temporary file and renaming only when completed?

[xzyfer]

That's certainly another alternative if we keep with the current approach of flushing to disk as data is received.

I've instead decided to go only flushing when the download is completed. Since the binary is only ~2MB I'm ok with leaving it memory until complete.