Add g++-4.8 to travis builds.
Add g++-4.8 to travis builds.

kleetus/bitcore-node,braydonf/bitcore-node,snogcel/bitcore-node-dash,kleetus/bitcore-node,snogcel/bitcore-node-dash,zcoinrocks/bitcore-node,braydonf/bitcore-node,zcoinrocks/bitcore-node