The latest version of adb is 1.0.39, however the version available from the Ubuntu repositories is 1.0.32. The repo version is dated March 2016, however ADB v1.0.32 was released when Lollipop launched.
Google recently made it easier to grab just the latest platform tools rather than download the whole Android SDK. I’ve written a small script to manually update
sudo cp platform-tools/adb /usr/bin/adb
sudo cp platform-tools/fastboot /usr/bin/fastboot
adb version to verify it’s been updated.
Copy & paste that into your favorite text editor like gedit or notepadqq, save it as a shell script and make it executable. When a new version of Android is released would be a good time to run it in case there’s new version of the platform tools.