spec_helper: improve exception handler

* print to stderr, not stdout
* rescue from StandardError, not Exception [1]

[1] http://goo.gl/r3nFG
spec_helper: avoid traceback with minitest 5

    $ bundle exec rake spec
    /usr/bin/ruby -S rspec spec/unit/chocolatey_spec.rb --color
    Warning: you should require 'minitest/autorun' instead.
    Warning: or add 'gem "minitest"' before 'require "minitest/autorun"'
spec_helper: avoid NameError exception on Minitest 5

Addresses:

    MiniTest::Unit::TestCase is now Minitest::Test. From
    /usr/share/ruby/test/unit/testcase.rb:8:in `<module:Unit>'
    ======================================================================
    NameError
    uninitialized class variable @@installed_at_exit in Test::Unit::Runner
    ======================================================================
Gemfile: require Minitest 5

Before this commit:

* fresh bundle gets Minitest 5
* existing bundle still has Minitest 4.x and
  may fail due to minitest/autorun in spec_helper

After this commit:

* all bundles get Minitest 5
Gemfile: Add travis and travis-lint

* Add travis gem to provide command-line interface, such as
  the `travis sync` command

* Add travis-lint gem to check validity of `.travis.yml` config
spec: add unit tests to validate .travis.yml

New unit tests:

    .travis.yml
      exists
      is a valid travis-ci configuration
travis: add current puppet versions to test matrix
travis: remove old puppet versions from test matrix
travis: add ruby 2.0 to test matrix

Before this commit: aa49dd787 removed ruby 2.0 due to test failure.

After this commit:

* Add ruby 2.0.

* We know puppet 2.7 does not support ruby 2.0, so
  exclude this combination from the test matrix.
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