(GH-57) Add ability to use specific version of Puppet Gem

Previously it was not possible to test on older puppet gems as the gemfile
always defaulted to latest.  This commit adds the PUPPET_GEM_VERSION
environment variable which can be used to set the version of the puppet gem
during a bundle.  This is the same workflow as regular Puppet modules.

Author: glennsarti

server/Gemfile

@@ -13,7 +13,11 @@ group :development do
   gem 'rspec', '>= 3.2',            :require => false
   gem "rubocop",                    :require => false, :platforms => [:ruby, :x64_mingw]
 
-  gem "puppet",                     :require => false
+  if ENV['PUPPET_GEM_VERSION']
+    gem 'puppet', ENV['PUPPET_GEM_VERSION'], :require => false
+  else
+    gem 'puppet',                            :require => false
+  end
 
   gem "win32-dir", "<= 0.4.9",      :require => false, :platforms => ["mswin", "mingw", "x64_mingw"]
   gem "win32-eventlog", "<= 0.6.5", :require => false, :platforms => ["mswin", "mingw", "x64_mingw"]