Change and reinstruct runtime dependency gem handling (#68)

Add a Gemfile with a reference only to 'gemspec'

Add runtime dependencies to liquid.gemspec so LD applications do not need to list them explicitly.

Reinstruct Gemfile usage in applications.

Gemfile

@@ -0,0 +1,3 @@
+source 'https://rubygems.org'
+
+gemspec

README.adoc

@@ -82,12 +82,6 @@ See https://rubyinstaller.org/downloads[rubyinstaller.org] if you're on Windows.
 ----
 source 'https://rubygems.org'
 
-gem 'json'
-gem 'liquid'
-gem 'asciidoctor'
-gem 'asciidoctor-pdf'
-gem 'logger'
-gem 'crack'
 gem 'liquidoc'
 ----
 +

liquidoc.gemspec

@@ -28,6 +28,19 @@ Gem::Specification.new do |spec|
   spec.executables   = ["liquidoc"]
   spec.require_paths = ["lib"]
 
+  spec.required_ruby_version     = ">= 2.3.0"
+  spec.required_rubygems_version = ">= 2.7.0"
+
   spec.add_development_dependency "bundler", "~> 1.15"
   spec.add_development_dependency "rake", "~> 10.0"
+
+  spec.add_runtime_dependency "asciidoctor", "~>1.5"
+  spec.add_runtime_dependency "json", "~>2.2"
+  spec.add_runtime_dependency "liquid", "~>4.0"
+  spec.add_runtime_dependency "asciidoctor-pdf", "~>1.5.0.alpha.16"
+  spec.add_runtime_dependency "logger", "~>1.3"
+  spec.add_runtime_dependency "crack", "~>0.4"
+  spec.add_runtime_dependency "jekyll", "~>3.0"
+  spec.add_runtime_dependency "jekyll-asciidoc", "~>2.1"
+  spec.add_runtime_dependency "highline", "~>2.0"
 end