Do not test under io.js (which is no longer a thing), test under Node.js
4.0.x and 5.0.x.
Zero minor versions are intentional, I wan't to make sure that PEG.js
doesn't depend on any features added later in the 4.x and 5.x cycle.
Travis CI builds with Node.js 0.8.x started to fail:
https://travis-ci.org/dmajda/pegjs/jobs/26691570
Rather than investigating what's wrong I decided to stop supporting Node
0.8.x. Node.js 0.10.x is here for over a year, which should be enough
time for everyone to upgrade in the fast-paced Node.js world.