Add icmp tests to travis, drop ruby 1.8.7, note known issues discovered in testing

* Add ICMP tests to travis-ci (refactored exclusions to work with rvmsudo);
* Drop ruby 1.8.7 test;
* Fix ping of 192.168.0.1 to use 127.0.0.1 instead;
* Move handling of EXCLUDE_ICMP_TEST to the same place as other excludes;
* Add to Known Issues in README based on testing results