travis: test against node v4 and v5
travis: test against node v4 and v5

Amok now targets Node v5, where-as the travis build matrix still targets
v0.11. v0.12 and iojs.

This updates the build matrix to target Node v4 and v5.

amokjs/amok,caspervonb/amok,amokjs/amok,caspervonb/amok,caspervonb/amok,amokjs/amok