Add minitest to development group

For Linter tests with 'bundle exec ruby test/test_linter.rb'.

Author: stomar

Gemfile

@@ -16,6 +16,7 @@ group :production do
 end
 
 group :development do
+  gem 'minitest'
   gem 'spidr', '~> 0.6'
   gem 'validate-website', '~> 1.6'
 end

Gemfile.lock

@@ -45,6 +45,7 @@ GEM
       ruby_dep (~> 1.2)
     mercenary (0.3.6)
     mini_portile2 (2.3.0)
+    minitest (5.11.3)
     nokogiri (1.8.5)
       mini_portile2 (~> 2.3.0)
     paint (1.0.1)
@@ -98,6 +99,7 @@ PLATFORMS
 DEPENDENCIES
   jekyll (~> 3.0)
   lanyon (~> 0.4.0)
+  minitest
   rack-protection
   rack-rewrite
   rack-ssl