Add -Dhalt.on.plugin.error=true to ant script

So that travis-CI will error out when a plugin fails to build.  The default
action is to silently ignore and continue building.

.travis.yml

 language: java
-script: ant -f build/build.xml openfire test plugins
+script: ant -Dhalt.on.plugin.error=true -f build/build.xml openfire test plugins