On the KeePassX site there is only a Linux version 0.4.3 for download.
how can I install the latest version on Ubuntu?
The problem is, that Version 0.4 is basically unmaintained.
I didn't have any trouble building from source:
In this Related bug report: Please update KeePassX to v2.0 Alpha
The developer now offers a PPA with daily builds for those who would like to test it: https://launchpad.net/~keepassx/+archive/daily