Don't use wp_convert_bytes_to_hr() deprecated function. Fixes #508.

As per http://core.trac.wordpress.org/ticket/19067, WordPress 3.6 deprecates the wp_convert_bytes_to_hr() function in favour of the size_format() (which was was added in WordPress 2.3).
Merge pull request #512 from om4james/wp_convert_bytes_to_hr

Don't use wp_convert_bytes_to_hr() deprecated function