Removed trailing whitespace from all *.rb files

Used this command:

```
find . -type f -name '*.rb' -exec sed --in-place 's/[[:space:]]\+$//' {} \+
```

as per first answer at
http://stackoverflow.com/questions/10711051/trailing-whitespace-elimination-script-for-multiple-files.

This will minimize or prevent conflicts when one of us whose editor automatically strips trailing white space does so.

URL's for comparing:

Ignoring whitespace:
https://github.com/alexdalitz/dnsruby/pull/19/files?w=1

Not ignoring whitespace:
https://github.com/alexdalitz/dnsruby/pull/19/files