Add Ruby 2.4 to the test matrix

Ensure that old rubies use Nokogiri 1.6.8
Fixes #1453

.travis.yml

@@ -4,9 +4,10 @@ language: ruby
 rvm:
   - 1.9.3
   - 2.0.0
-  - 2.1.5
-  - 2.2.4
-  - 2.3.0
+  - 2.1.10
+  - 2.2.6
+  - 2.3.3
+  - 2.4.0
   - ruby-head
   - jruby
 gemfile:
@@ -23,15 +24,19 @@ matrix:
       gemfile: Gemfile
     - rvm: 2.0.0
       gemfile: Gemfile
-    - rvm: 2.1.5
+    - rvm: 2.1.10
       gemfile: Gemfile
+    - rvm: 2.4.0
+      gemfile: gemfiles/Gemfile.rails-4-1-stable # activesupport 4.1 depends on json 1.8.3 which does not compile with Ruby 2.4.0
+    - rvm: ruby-head
+      gemfile: gemfiles/Gemfile.rails-4-1-stable # activesupport 4.1 depends on json 1.8.3 which does not compile with Ruby 2.4.0
     - rvm: jruby
       gemfile: Gemfile
     - rvm: 1.9.3
       gemfile: gemfiles/Gemfile.rails-5-0-stable
     - rvm: 2.0.0
       gemfile: gemfiles/Gemfile.rails-5-0-stable
-    - rvm: 2.1.5
+    - rvm: 2.1.10
       gemfile: gemfiles/Gemfile.rails-5-0-stable
     - rvm: jruby
       gemfile: gemfiles/Gemfile.rails-5-0-stable

gemfiles/Gemfile.rails-4-0-stable

@@ -14,3 +14,4 @@ gem 'actionpack', github: 'rails/rails', branch: '4-0-stable'
 gem 'rake'
 gem 'tzinfo'
 gem 'rdoc'
+gem 'nokogiri', '~> 1.6.8'

gemfiles/Gemfile.rails-4-1-stable

@@ -13,3 +13,4 @@ gem 'activemodel', github: 'rails/rails', branch: '4-1-stable'
 gem 'actionpack', github: 'rails/rails', branch: '4-1-stable'
 gem 'rake'
 gem 'tzinfo'
+gem 'nokogiri', '~> 1.6.8'

gemfiles/Gemfile.rails-4-2-stable

@@ -13,3 +13,4 @@ gem 'activemodel', github: 'rails/rails', branch: '4-2-stable'
 gem 'actionpack', github: 'rails/rails', branch: '4-2-stable'
 gem 'rake'
 gem 'tzinfo'
+gem 'nokogiri', '~> 1.6.8'