Adding support for `nvm download` behind a proxy.

If you are behind a proxy and you still want to be able to download node versions, you just need to edit (add) the following file to your HOME path : `.nvmrc`

```
# content of ~/.nvmrc

proxy = http://X.X.X.X:8080/

```
Update nvm-download
Merge pull request #1 from Martin-Luther/patch-1

Adding support for `nvm download` behind a proxy.