Fix the way that el-get accesses `url-proxy-services`.

el-get gets the proxy from `url-proxy-services` by

    (cadr (assoc "http" url-proxy-services))

However, `url-proxy-services` is an alist which looks like

    '(("http" . "hostname:portnumber') ...)

So using `cadr` is wrong.  Replace it with `cdr`.

This commit closes dimitri#1615