Tagged and released version 2.3.1

git-svn-id: https://plugins.svn.wordpress.org/pressbooks/trunk@951238 b8457f37-d9ea-0310-8a92-e5e31aec5664
Tagged and released version 2.3.1, with corrected version number.

git-svn-id: https://plugins.svn.wordpress.org/pressbooks/trunk@951239 b8457f37-d9ea-0310-8a92-e5e31aec5664