Remove border override for paper-dialog content

See http://www.dartosphere.org/2014/08/25/at-the-bottom-of-the-rabbit-hole-i-find-polyfill-next-selector.html#.U_yWPXWx3UY. I tested it out briefly and don't see a reason for this border: 0; style either. If there is something specific that is trying to be targeted it should probably be put in a less general selector?
Merge pull request #22 from jakemac53/patch-1

remove border override on content