Use TLS (HTTPS) to retrieve composer.phar

Set the download URL to use HTTPS, which provides some guarantee as to
the authenticity of the file being downloaded.

HTTPS is supported by getcomposer.org:

    » curl -I -XHEAD https://getcomposer.org/ 2>/dev/null | head -n 1
    » HTTP/1.1 200 OK
Merge pull request #56 from mattbostock/master

Use TLS (HTTPS) to retrieve composer.phar