Update node versions tested on CI
Update node versions tested on CI

* Enable testing on iojs and node 0.12 (since we support both)
* Stop testing on node < 0.12 (since that's no longer supported)

amitmotgi/node-http2,mareksrom/node-http2,5lava/node-http2,rmurphey/node-http2,jstourac/node-http2,nwgh/node-http2,scoky/node-http2,scoky/node-http2,oimou/node-http2,molnarg/node-http2,tbetbetbe/node-http2