Fix Makefile error when downloading Python

Because cURL is not told to follow redirects (with a --location option), it fails to download Python.

The redirects that Python.org returns is a 301 (Moved Permanently).

Here is the flow of redirects:

http://www.python.org/ftp/python/2.7.3/Python-2.7.3.tgz 301
-> https://www.python.org/ftp/python/2.7.3/Python-2.7.3.tgz 301
-> http://legacy.python.org/ftp//python/2.7.3/Python-2.7.3.tgz

To honor the redirects, I've changed the download URL to point to the final URL: http://legacy.python.org/ftp//python/2.7.3/Python-2.7.3.tgz. This change fixes the Makefile and unblocks Travis CI to run integration tests.