Require Node.js >= 0.10.0

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.