Correctly map curl's `-C -` to wget

wget doesn't need or accept the `-` parameter to `-c`

Without it, on a `curl`-less OS `nvm install` doesn't work since wget tries to download from `http://-/` apart from the real Node file which causes the process to fail and makes nvm think the binary download didn't succeed.

So this is pretty critical to wget users.

Should wget `nvm install` be tested somehow? If it was, this issue wouldn't happen.