Add explicit test for the active_resource 5+.

.travis.yml

@@ -11,6 +11,8 @@ gemfile:
   - Gemfile_ar31
   - Gemfile_ar32
   - Gemfile_ar40
+  - Gemfile_ar50
+  - Gemfile_ar51
   - Gemfile_ar_master
 
 matrix:
@@ -26,6 +28,10 @@ matrix:
       gemfile: Gemfile_ar32
     - rvm: 2.4.0-preview1
       gemfile: Gemfile_ar40
+    - rvm: 2.4.0-preview1
+      gemfile: Gemfile_ar50
+    - rvm: 2.4.0-preview1
+      gemfile: Gemfile_ar51
     - rvm: 2.4.0-preview1
       gemfile: Gemfile_ar_master
     - rvm: 2.3.1

Gemfile_ar50

@@ -0,0 +1,6 @@
+source "https://rubygems.org"
+
+gemspec
+
+gem "activeresource", "5.0.0"
+gem "rack", "< 2" if RUBY_VERSION < "2.2"

Gemfile_ar51

@@ -0,0 +1,6 @@
+source "https://rubygems.org"
+
+gemspec
+
+gem "activeresource", "5.1.0"
+gem "rack", "< 2" if RUBY_VERSION < "2.2"