changed md5 to sha256

brew has started to complain that the md5 hash validation was going to be deprecated in a future version. 

I pulled the tarball, did an md5 to validate it was the same that you assumed, then did an shasum -a256 and generated the SHA256 hash.  Replaced the md5 hash with the sha256 hash in the imapsync.rb file.  

Tested it all and it works.  

BTW - why has this never been merged into the main homebrew distro?

Christopher