Improve test environment

* Add Rakefile, Gemfile, .travis.yml.
* Add a list of offcially supported Ruby to .travis.yml.
* Add ruby-head to Travis as allow_failures.
  allow_failures is because it's good to know new version Ruby's issue
  as faster before the release.
* fast_finish is to get the Travis result as faster
  without waiting the result of the "allow_failures" items.
  See https://blog.travis-ci.com/2013-11-27-fast-finishing-builds/

.travis.yml

@@ -0,0 +1,19 @@
+language: ruby
+
+sudo: false
+
+before_install:
+  - gem install bundler
+
+rvm:
+  - 2.0.0
+  - 2.1.10
+  - 2.2.6
+  - 2.3.3
+  - 2.4.1
+  - ruby-head
+
+matrix:
+  allow_failures:
+    - rvm: ruby-head
+  fast_finish: true

Gemfile

@@ -0,0 +1,7 @@
+source "https://rubygems.org"
+
+gemspec
+
+gem 'rake', '~> 12.0.0'
+gem 'test-unit', '~> 3.2.3'
+gem 'minitest', '~> 5.10.1'

Rakefile

@@ -0,0 +1,10 @@
+require 'rake/testtask'
+
+Rake::TestTask.new(:test) do |t|
+  t.libs << "test"
+  t.test_files = FileList["test/**/*_test.rb"]
+  t.warning = true
+  t.verbose = true
+end
+
+task :default => :test