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

@@ -10,6 +10,7 @@ rvm:
  - 2.2.6
  - 2.3.3
  - 2.4.0
+ - ruby-head
  - jruby
 gemfile:
  - Gemfile
@@ -20,6 +21,9 @@ matrix:
  - rvm: jruby
  gemfile: gemfiles/Gemfile.minitest.latest
  env: MOCHA_OPTIONS=debug MOCHA_RUN_INTEGRATION_TESTS=minitest
+ - rvm: ruby-head
+ gemfile: gemfiles/Gemfile.minitest.latest
+ env: MOCHA_OPTIONS=debug MOCHA_RUN_INTEGRATION_TESTS=minitest
  - rvm: 2.4.0
  gemfile: gemfiles/Gemfile.minitest.latest
  env: MOCHA_OPTIONS=debug MOCHA_RUN_INTEGRATION_TESTS=minitest
@@ -86,6 +90,9 @@ matrix:
  - rvm: jruby
  gemfile: gemfiles/Gemfile.test-unit.latest
  env: MOCHA_OPTIONS=debug MOCHA_RUN_INTEGRATION_TESTS=test-unit
+ - rvm: ruby-head
+ gemfile: gemfiles/Gemfile.test-unit.latest
+ env: MOCHA_OPTIONS=debug MOCHA_RUN_INTEGRATION_TESTS=test-unit
  - rvm: 2.4.0
  gemfile: gemfiles/Gemfile.test-unit.latest
  env: MOCHA_OPTIONS=debug MOCHA_RUN_INTEGRATION_TESTS=test-unit
@@ -128,3 +135,6 @@ matrix:
  - rvm: 1.8.7-p371
  gemfile: Gemfile
  env: MOCHA_OPTIONS=debug MOCHA_RUN_INTEGRATION_TESTS=test-unit
+ allow_failures:
+ - rvm: ruby-head
+ fast_finish: true