Fix threecol layout in Roundcube 1.0.x.

The base template in skins/larry has changed, so the evil
regexp substiution on the full page content in this plugin
needs to be updated to match.

It might make sense to have two sets of substitutions depending
on the Roundcube version.
Merge pull request #6 from dpc22/master

Fix threecol layout in Roundcube 1.0.x.