pom.xml: revert eventtracker to 3.1.0
pom.xml: revert eventtracker to 3.1.0

There are a lot of changes coming in from eventtracker but they are not
quite ready yet.

Signed-off-by: Pierre-Alexandre Meyer <ff019a5748a52b5641624af88a54a2f0e46a9fb5@ning.com>

pierre/collector,pierre/collector,ning/collector,ning/collector,ning/collector,ning/collector,pierre/collector,pierre/collector,pierre/collector,ning/collector,pierre/collector