Updated Picard packages for Ubuntu are available over the official MusicBrainz Ubuntu PPAs. For most users it is recommended to use the stable PPA to install the latest stable release of Picard (1.4.2 at the time of writing this):
sudo add-apt-repository ppa:musicbrainz-developers/stable sudo apt update sudo apt install picard
Up-to-date stable packages are available for Ubuntu 14.04 Trusty, 15.05 Vivid, 16.04 Xenial, 17.04 Zesty and upcoming 17.10 Artful.
For users wanting to try out the latest development release there is also a daily build PPA available. Please note that the current Picard 2.0 development releases are a major update to the codebase and everything is still work in progress, so it might be more unstable then usually. Also due to updated dependencies the Picard 2 daily builds are only available for Ubuntu 17.04 and 17.10.
More details can be found in this post on the MetaBrainz Community.