Enable tests with Ruby 2.2

Adds rubysl-test-unit for Rails 4. See issue:
rspec/rspec-rails#858

rubysl-test-unit avoids failures:
https://travis-ci.org/doorkeeper-gem/doorkeeper/builds/50356279

Allows failures for Rails 3.2 on Ruby 2.2, as it's not supported.

.travis.yml

@@ -5,6 +5,7 @@ cache: bundler
 rvm:
   - 2.0
   - 2.1
+  - 2.2
 
 env:
   - rails=3.2.0
@@ -23,6 +24,9 @@ services:
 
 matrix:
   exclude:
+    - env: rails=3.2.0
+      rvm: 2.2
+
     - gemfile: gemfiles/Gemfile.mongoid2.rb
       env: rails=4.1.0
     - gemfile: gemfiles/Gemfile.mongoid2.rb

Gemfile

@@ -7,5 +7,8 @@ gem 'rails', "~> #{ENV['rails']}"
 if ENV['rails'][0] == '4'
   gem 'database_cleaner'
 end
+if ENV['rails'] =~ /4.0|3.2/
+  gem 'rubysl-test-unit'
+end
 
 gemspec

gemfiles/Gemfile.common.rb

@@ -7,5 +7,8 @@
 if ENV['rails'][0] == '4'
   gem 'database_cleaner', '~> 1.3.0'
 end
+if ENV['rails'] =~ /4.0|3.2/
+  gem 'rubysl-test-unit'
+end
 
 gemspec path: '../'