Tell proxy to use ssl

It appears that proxy doesn't use ssl unless you tell it to use_ssl in proxy.start.

Otherwise I get a 400 error when downloading artifacts like the following:

```
FAILURE (node/lucid/x86_64/node-0.9.11.tar.gz): Unable to download from 'https://download.run.pivotal.io/node/lucid/x86_64/node-0.9.11.tar.gz'.  Received '400'.
```