Upgrade to Velocity 1.5 Beta 2

Even though Velocity 1.5 Beta2 is a beta and not final, we should move to it. The reasons:

1) Lots of things have been fixed and interesting improvements and new features are available. See http://wiki.apache.org/velocity/Velocity15ReleaseNotes
2) This fixes an important security issue with XWiki: XWIKI-457
3) As we want to use Velocity 1.5 for the 1.0 final version of XWiki we need to test it ASAP during our beta cycle to see if there are any issues. If any we'll have the time to raise them against the Velocity team. If we push the move to later, it'll be too late. We can always revert to 1.4 if we find big issues.