invalid core gems creation

for the 2.2 release snapshot we ran into a problem where the core gems (logtash-core and logstash-core-event) were incorrectly built and the problem was that the `gem build` command was launched from the logstash root and not from the core gems directories. For gems to build correctly, the `gem build` command must be launched from the respective gem homes.

to avoid this in the future we should add a rake task to build the core gems.