lib: errno is not a global in node >= v0.9.11

Update the global leak checker to add errno to the list of expected
globals only when the node version is < v0.9.11.
Merge pull request #1111 from bnoordhuis/report-errno-global-leaks

lib: errno is not a global in node >= v0.9.11

* bnoordhuis/report-errno-global-leaks:
  lib: errno is not a global in node >= v0.9.11
Cleanup #1111