Fix `cabal-install/bootstrap.sh` for the new Hackage server

The new Hackage server uses slightly different URLs for packages, and it returns 301 redirects for the old URLs. The current invocation of `curl` in `bootstrap.sh` doesn't follow redirects:

    $ curl -v --fail -C - -O http://hackage.haskell.org/packages/archive/Cabal/1.18.0/Cabal-1.18.0.tar.gz
    * About to connect() to hackage.haskell.org port 80 (#0)
    *   Trying 88.198.224.242...
      % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                     Dload  Upload   Total   Spent    Left  Speed
      0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0* connected
    * Connected to hackage.haskell.org (88.198.224.242) port 80 (#0)
    > GET /packages/archive/Cabal/1.18.0/Cabal-1.18.0.tar.gz HTTP/1.1
    > User-Agent: curl/7.24.0 (x86_64-apple-darwin12.0) libcurl/7.24.0 OpenSSL/0.9.8y zlib/1.2.5
    > Host: hackage.haskell.org
    > Accept: */*
    >
    < HTTP/1.1 301 Moved Permanently
    < Server: nginx/1.4.2
    < Date: Fri, 27 Sep 2013 02:14:17 GMT
    < Content-Type: text/plain; charset=UTF-8
    < Transfer-Encoding: chunked
    < Connection: keep-alive
    < Location: /package/Cabal-1.18.0/Cabal-1.18.0.tar.gz
    <
    { [data not shown]
      0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
    * Connection #0 to host hackage.haskell.org left intact
    * Closing connection #0

Adding the `-L` flag causes `curl` to follow redirects and get the packages as intended.

Note that `wget` (the second-choice downloader) follows redirects automatically. I'm not sure about `fetch`.