Test against Jekyll 4.x (#336)

Merge pull request 336

.rubocop.yml

@@ -1,3 +1,5 @@
+inherit_from: .rubocop_todo.yml
+
 require: rubocop-jekyll
 inherit_gem:
   rubocop-jekyll: .rubocop.yml

.rubocop_todo.yml

@@ -0,0 +1,13 @@
+# This configuration was generated by
+# `rubocop --auto-gen-config`
+# on 2019-03-25 11:42:06 +0100 using RuboCop version 0.66.0.
+# The point is for the user to remove these configuration records
+# one by one as the offenses are removed from the code base.
+# Note that changes in the inspected code, or installation of new
+# versions of RuboCop, may require this file to be generated again.
+
+# Offense count: 1
+# Cop supports --auto-correct.
+Lint/ToJSON:
+  Exclude:
+    - 'lib/jekyll-seo-tag/json_ld_drop.rb'

.travis.yml

@@ -10,3 +10,6 @@ script: script/cibuild
 env:
   global:
     - NOKOGIRI_USE_SYSTEM_LIBRARIES=true
+  matrix:
+    - JEKYLL_VERSION="~> 3.3"
+    - JEKYLL_VERSION=">= 4.0.0.pre.alpha1"

Gemfile

@@ -4,4 +4,4 @@ source "https://rubygems.org"
 
 gemspec
 
-gem "github-pages", :group => :jekyll_plugins
+gem "jekyll", ENV["JEKYLL_VERSION"] if ENV["JEKYLL_VERSION"]

jekyll-seo-tag.gemspec

@@ -28,7 +28,7 @@ Gem::Specification.new do |spec|
   spec.executables   = spec.files.grep(%r!^exe/!) { |f| File.basename(f) }
   spec.require_paths = ["lib"]
 
-  spec.add_dependency "jekyll", "~> 3.3"
+  spec.add_dependency "jekyll", ">= 3.3", "< 5.0"
   spec.add_development_dependency "bundler", ">= 1.15"
   spec.add_development_dependency "html-proofer", "~> 3.7"
   spec.add_development_dependency "rspec", "~> 3.5"