Use TLS (HTTPS) to retrieve composer.phar

Set the default of the `$phar_location` location 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