Guava version conflict

[raboof]

I've upgraded Xebium to use Selenium 2.38.0.

This version of Selenium has upgraded guava from 0.14 to 0.15. Unfortunately it appears guava 0.15 makes our FitNesse tests fail to start.

Explicitly downgrading to 0.14 in the xebium pom.xml works around that problem, but it'd be better to find a more permanent solution.