travis: add ruby 2.0 to test matrix

Before this commit: Travis runs 4 tests on each pull request...

    Rvm   Env
    1.8.7 PUPPET_VERSION=2.7.22
    1.8.7 PUPPET_VERSION=3.2.2
    1.9.3 PUPPET_VERSION=2.7.22
    1.9.3 PUPPET_VERSION=3.2.2

After this commit: Travis runs 5 tests on each pull request...

    Rvm   Env
    1.8.7 PUPPET_VERSION=2.7.22
    1.8.7 PUPPET_VERSION=3.2.2
    1.9.3 PUPPET_VERSION=2.7.22
    1.9.3 PUPPET_VERSION=3.2.2
    2.0.0 PUPPET_VERSION=3.2.2
git: show smarter diff for ruby files
git: ensure ruby files have LF line endings
README: improve markdown formatting

* Use syntax highlighting for puppet example
* Use literal tags for inline samples
* Add links to chocolatey and puppet web sites
README: show how to set chocolatey as default provider
CONTRIBUTING: add instructions to contribute

This doc is shown to people who open a pull request on Github
as described at https://github.com/blog/1184-contributing-guidelines
README: Use correct upstream build status for travis badge

@rismoney transferred the upstream repo to the chocolatey org
a few months ago. It makes sense that the build status should
be badged against the upstream org.
spec: protect against trailing whitespace

Add unit test and convert README with dos2unix line endings
to make the test pass.

    git commits
      should not introduce trailing whitespace