use rake_cmd instead of 'heroku rake'

cedar uses a 'heroku run rake' instead of 'heroku rake'.
Fixes jacquescrocker#10

lib/heroku/rails/tasks.rb

@@ -62,10 +62,12 @@ def system_with_echo(*args)
       Rake::Task["heroku:before_each_deploy"].reenable
       Rake::Task["heroku:before_each_deploy"].invoke(app_name)
 
+      rake_cmd = HEROKU_CONFIG.rake_cmd(heroku_env)
+
       branch = `git branch`.scan(/^\* (.*)\n/).flatten.first.to_s
       if branch.present?
         @git_push_arguments ||= []
-        system_with_echo "git push #{repo} #{@git_push_arguments.join(' ')} #{branch}:master && heroku rake --app #{app_name} db:migrate && heroku restart --app #{app_name}"
+        system_with_echo "git push #{repo} #{@git_push_arguments.join(' ')} #{branch}:master && #{rake_cmd} --app #{app_name} db:migrate && heroku restart --app #{app_name}"
       else
         puts "Unable to determine the current git branch, please checkout the branch you'd like to deploy"
         exit(1)
@@ -179,7 +181,8 @@ def system_with_echo(*args)
     desc "Migrates and restarts remote servers"
     task :migrate do
       HEROKU_RUNNER.each_heroku_app do |heroku_env, app_name, repo|
-        system_with_echo "heroku rake --app #{app_name} db:migrate && heroku restart --app #{app_name}"
+        rake_cmd = HEROKU_CONFIG.rake_cmd(heroku_env)
+        system_with_echo "#{rake_cmd} --app #{app_name} db:migrate && heroku restart --app #{app_name}"
       end
     end
 
@@ -200,4 +203,4 @@ def system_with_echo(*args)
       end
     end
   end
-end
+end