Incremented version to 1.56.1.  Changed spec.files to use git ls-files.

Changes to files included in gem are:

➜  gems  diff 0.dir 1.dir
1a2,4
> ./.coveralls.yml
> ./.gitignore
> ./.travis.yml
13a17
> ./dnsruby.gemspec
16a21
> ./Gemfile
91a97
> ./NEWS
93a100,101
> ./RELEASE_NOTES.md
> ./SIGNED_UPDATES
110d117
< ./test/tc_keith.rb