update plexus-utils to avoid leaking processes in CommandLineUtils.getSystemEnvars()

Not long ago, it was discovered that CommandLineUtils leaks processes in come circumstances (in older versions of plexus-utils) because it fails to read stderr. Later versions of plexus-utils (1.5.11 and above) have fixed this problem.

Interestingly, Hudson has had a persistent problem with leaking processes on the Windows slave VM out on https://grid.sonatype.org/ci/ - I'm pretty sure this will help alleviate those issues. We should move up to the latest version of plexus-utils to take advantage of this bugfix.