Node 0.6 execution fix

`bin/semver-sync` used to fail running on Node.js 0.6. I discovered this when I added the execution of `semver-sync` as part of my `test` target in a Makefile of mine and Travis [failed](https://travis-ci.org/lddubeau/salve/builds/7632541) to build my code in 0.6 where previously there was no problem.

So here is the fix. What I did:

* Added a test to actually run `bin/semver-sync`. I then checked that this test would fail in 0.6. ([Here](https://travis-ci.org/mangalam-research/semver-sync/builds/7641128) is the Travis build showing failure.)
* Fixed `bin/semver-sync`. Checked that the build would pass on Travis.
* Updated `.travis.yml` to also test on Node.js 0.10 and 0.11. Node.js 0.10 is out and Node.js 0.11 is the development version. Testing against it is prudent to prevent breakage when 0.12 comes out.