Make tests run in Ruby 2.1.x and add Travis CI support.

Tests were failing to run in Ruby 2.1.x with the following error:

```
rgrove@sulaco:~/src/nokogumbo (master)$ rake
/Users/rgrove/.rbenv/versions/2.1.0/bin/ruby test-nokogumbo.rb
Warning: you should require 'minitest/autorun' instead.
Warning: or add 'gem "minitest"' before 'require "minitest/autorun"'
From:
  /Users/rgrove/.rbenv/versions/2.1.0/lib/ruby/2.1.0/test/unit.rb:1:in `<top (required)>'
  test-nokogumbo.rb:5:in `<main>'
MiniTest::Unit::TestCase is now Minitest::Test. From /Users/rgrove/.rbenv/versions/2.1.0/lib/ruby/2.1.0/test/unit/testcase.rb:8:in `<module:Unit>'
/Users/rgrove/.rbenv/versions/2.1.0/lib/ruby/2.1.0/test/unit.rb:676:in `<class:Runner>': undefined method `_run_suite' for class `Test::Unit::Runner' (NameError)
	from /Users/rgrove/.rbenv/versions/2.1.0/lib/ruby/2.1.0/test/unit.rb:261:in `<module:Unit>'
	from /Users/rgrove/.rbenv/versions/2.1.0/lib/ruby/2.1.0/test/unit.rb:15:in `<module:Test>'
	from /Users/rgrove/.rbenv/versions/2.1.0/lib/ruby/2.1.0/test/unit.rb:7:in `<top (required)>'
	from /Users/rgrove/.rbenv/versions/2.1.0/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require'
	from /Users/rgrove/.rbenv/versions/2.1.0/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require'
	from test-nokogumbo.rb:5:in `<main>'
rake aborted!
Command failed with status (1): [/Users/rgrove/.rbenv/versions/2.1.0/bin/ru...]
/Users/rgrove/src/nokogumbo/Rakefile:11:in `block in <top (required)>'
Tasks: TOP => default => test
(See full trace by running task with --trace)
```

This PR fixes that, and also adds a Travis CI config to allow automated testing. If you merge this PR, you can enable Travis by adding its GitHub webhook as described here: http://docs.travis-ci.com/user/getting-started/

If you're not interested in using Travis, let me know and I'll open a new PR without the Travis commit.