Fixed timeouts in Net::Ping::External

Using Timeout.timeout with Open3 is not an option with Ruby > 1.9.3
(see https://bugs.ruby-lang.org/issues/5487). Implemented a workaround
based on the code at https://gist.github.com/lpar/1032297
Update travis.yml to trigger CI run

This update should trigger a Travis CI build on different Rubies.
Merge pull request #32 from bcandrea/develop

Fixed timeouts in Net::Ping::External