Fixed curl silently ignoring 302 redirections

Under certain network environments, due to poor implementation of file
download caches (immoral Chinese ISP), the direct download of Node.js
packages (http://nodejs.org/dist/node-$VERSION.tar.gz) will be
redirected to alternative urls using 302 redirections, which are not
handled by `curl --process-bar` by default. Instead, curl will "fail"
silently without creating any output file or error exitcode.
(Tested under Ubuntu 12.04 Desktop)

Fixed by adding "-L" switches to the curl commands responsible for
downloading the binary and source node.js packages.