Make Vox Rake tasks conditional

This commit updates the Rakefile to only attempt to use Vox Rake tasks
if the voxpupuli-acceptance gem is installed.

Author: mhashizume

.sync.yml

@@ -42,4 +42,5 @@ appveyor.yml:
 # We still use the Vox acceptance Rake task instead of Litmus
 Rakefile:
   requires:
-    - voxpupuli/acceptance/rake
+    - require: voxpupuli/acceptance/rake
+      conditional: Gem.loaded_specs.key? 'voxpupuli-acceptance'

Rakefile

@@ -6,7 +6,7 @@ require 'puppetlabs_spec_helper/rake_tasks'
 require 'puppet-syntax/tasks/puppet-syntax'
 require 'github_changelog_generator/task' if Gem.loaded_specs.key? 'github_changelog_generator'
 require 'puppet-strings/tasks' if Gem.loaded_specs.key? 'puppet-strings'
-require 'voxpupuli/acceptance/rake'
+require 'voxpupuli/acceptance/rake' if Gem.loaded_specs.key? 'voxpupuli-acceptance'
 
 def changelog_user
   return unless Rake.application.top_level_tasks.include? "changelog"