Allow deploying a specific branch to production

We could also use the deploy-secrets file; here we use `ENV["branch"]`
for consistency with other environments.

config/deploy/production.rb

@@ -1,4 +1,4 @@
-set :branch, :master
+set :branch, ENV["branch"] || :master
 
 server deploysecret(:server1), user: deploysecret(:user), roles: %w[web app db importer cron background]
 #server deploysecret(:server2), user: deploysecret(:user), roles: %w(web app db importer cron background)