Add .travis.yml (#161)

* Add supported Rubies.
* Add ruby-head as allow_failures.
  * 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/
* Add Gemfile.lock to .gitignore.

.gitignore

@@ -1,6 +1,7 @@
 pkg
 doc
 coverage
+Gemfile.lock
 VERSION
 *.rbc
-.bundle
+.bundle

.travis.yml

@@ -0,0 +1,16 @@
+language: ruby
+sudo: false
+before_install:
+  - which bundle || gem install bundler
+rvm:
+  - 2.2.7
+  - 2.3.4
+  - 2.4.1
+  - ruby-head
+  - jruby-9.1.8.0
+  - jruby-head
+matrix:
+  allow_failures:
+    - rvm: ruby-head
+    - rvm: jruby-head
+  fast_finish: true