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