Changed the 'validate_download' subprocess to evaluate whether
or not the user has Digest::MD5 installed, and to die if they
don't.

Also removed the lower-case conversion of the MD5 string provided by YNAB;
It seems better to keep that value as intact as possible. The calculated
MD5 is now converted to upper-case during the comparison process.

Signed-off-by: Elliot Voris <elliotfriend@gmail.com>