Allow testing with Jekyll 4.x (jekyll#351)

Author: ashmaroli

.travis.yml

@@ -1,6 +1,15 @@
 language: ruby
 cache: bundler
-rvm: 2.4
+rvm:
+  - 2.6
+  - 2.4
 
+before_install: gem update --system
 install: script/bootstrap
 script: script/cibuild
+
+env:
+  matrix:
+    - JEKYLL_VERSION="~> 3.5"
+    - JEKYLL_VERSION="~> 3.8.0"
+    - JEKYLL_VERSION=">= 4.0.0.pre.alpha1"

Gemfile

@@ -2,3 +2,5 @@
 
 source "https://rubygems.org"
 gemspec
+
+gem "jekyll", ENV["JEKYLL_VERSION"] if ENV["JEKYLL_VERSION"]

minima.gemspec

@@ -16,7 +16,7 @@ Gem::Specification.new do |spec|
     f.match(%r!^(assets|_(includes|layouts|sass)/|(LICENSE|README)((\.(txt|md|markdown)|$)))!i)
   end
 
-  spec.add_runtime_dependency "jekyll", "~> 3.5"
+  spec.add_runtime_dependency "jekyll", ">= 3.5", "< 5.0"
   spec.add_runtime_dependency "jekyll-feed", "~> 0.9"
   spec.add_runtime_dependency "jekyll-seo-tag", "~> 2.1"