Don't use id for flash view

When logging out and then logging back in, there is sometimes an error
when registering flash view, which says that it can't register 2 views
with the same id. We don't need to use id here, so I just changed id to
class

assets/scripts/app/views/flash.coffee

@@ -1,6 +1,6 @@
 @Travis.reopen
   FlashView: Travis.View.extend
-    elementId: 'flash'
+    classNames: ['flash']
     tagName: 'ul'
     templateName: 'layouts/flash'
 

assets/styles/app/flash.sass

@@ -1,6 +1,6 @@
 @import "_mixins/all"
 
-#flash
+.flash
   color: gray
   font-weight: bold