Update note about SemVer in readme.
Add travis badge and update ruby testing info.
Refactor out rexml and nokogiri xml support.

Because we use ratom and it uses libxml we will always have libxml so just
always use that.
Add ruby 1.8.7 back into the travis config.

Now we no longer use nokogiri we are good for 1.8.7 again, I think.