Fix icmp enetunreach bug

This fixes the ICMP bug when pinging unreachable hosts - instead of throwing an exception it now returns false. The travis config has been updated to expect the bug to be fixed.

This branch is based on #35 extra_timeout_and_unreachable_tests pull request.

I hope you don't feel pressured with (yet) another pull request. Are you happy for me to submit further pull requests for other bugs fixes / enhancements or do you want me to wait till you have digested this and the previous pull request?