Can't use it in Ubuntu18.04

[bedwardly-down]

@CalebJohn

Request: Implementing MD5 Checksum Into The Joplin Build Process

After a user had an error come up due to their file not being fully downloaded properly using the Joplin_install_and_update.sh script, I took a look at it and noticed there was no way to check if the download was properly downloaded correctly. MD5 Checksums are a quick and easy way to verify a download without having to add too much to a project and are pretty standard across the board.