Lita config is frozen since 3.0.0 and xmpp4r will try to downcase! @domain

Fix for
```
/u/apps/lita/vendor/ruby/2.1.0/gems/xmpp4r-0.5.6/lib/xmpp4r/jid.rb:41:in `downcase!': can't modify frozen String (RuntimeError)
	from /u/apps/lita/vendor/ruby/2.1.0/gems/xmpp4r-0.5.6/lib/xmpp4r/jid.rb:41:in `initialize'
	from /u/apps/lita/vendor/ruby/2.1.0/gems/xmpp4r-0.5.6/lib/xmpp4r/jid.rb:67:in `new'
	from /u/apps/lita/vendor/ruby/2.1.0/gems/xmpp4r-0.5.6/lib/xmpp4r/jid.rb:67:in `strip'
	from /u/apps/lita/vendor/ruby/2.1.0/bundler/gems/lita-xmpp-50008c2b973b/lib/lita/adapters/xmpp/connector.rb:43:in `block in join_rooms'
	from /u/apps/lita/vendor/ruby/2.1.0/bundler/gems/lita-xmpp-50008c2b973b/lib/lita/adapters/xmpp/connector.rb:40:in `each'
	from /u/apps/lita/vendor/ruby/2.1.0/bundler/gems/lita-xmpp-50008c2b973b/lib/lita/adapters/xmpp/connector.rb:40:in `join_rooms'
	from /u/apps/lita/vendor/ruby/2.1.0/bundler/gems/lita-xmpp-50008c2b973b/lib/lita/adapters/xmpp.rb:27:in `run'
	from /u/apps/lita/vendor/ruby/2.1.0/gems/lita-3.3.1/lib/lita/robot.rb:47:in `run'
	from /u/apps/lita/vendor/ruby/2.1.0/gems/lita-3.3.1/lib/lita.rb:141:in `run'
	from /u/apps/lita/vendor/ruby/2.1.0/gems/lita-3.3.1/lib/lita/cli.rb:75:in `start'
	from /u/apps/lita/vendor/ruby/2.1.0/gems/thor-0.19.1/lib/thor/command.rb:27:in `run'
	from /u/apps/lita/vendor/ruby/2.1.0/gems/thor-0.19.1/lib/thor/invocation.rb:126:in `invoke_command'
	from /u/apps/lita/vendor/ruby/2.1.0/gems/thor-0.19.1/lib/thor.rb:359:in `dispatch'
	from /u/apps/lita/vendor/ruby/2.1.0/gems/thor-0.19.1/lib/thor/base.rb:440:in `start'
	from /u/apps/lita/vendor/ruby/2.1.0/gems/lita-3.3.1/bin/lita:6:in `<top (required)>'
	from /u/apps/lita/bin/lita:16:in `load'
	from /u/apps/lita/bin/lita:16:in `<main>'
```